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The Status of Carbon, Iron, 
and Sulphur in Clays, 

DURING THE 

Various Stages of Burning. 




Third Report of Committee on Technical Investigation. 


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National Brick Manufacturers’ 

H 

Association 

—OF THE — 

UNITED STATES OF AMERICA 


A Study of the Chemical Status of the Carbon. Iron, 
and Sulphur in Clays, during the various stages 

of Burning. 


X 


Being the Third Report of the Committee on 
Technical Investigation. 


PERSONNEL OF THE COMMITTEE: 

Prof. Edward Orton, Jr., Columbus, Ohio.Permanent Chairman 


D. V. Purington, Chicago, Ill.Term expires 1908 

W. D. Richardson, Columbus, Ohio.Term expires 1910 

C. A. Bloomfield, Metuchen, N. J.Term expires 1909 

Anthony Ittner, St Louis, Mo. Term expires 1911 


Homer H. Staley, B. Sc .Uniontown, Pa. 


Scholarship Appointee, 1904-1905. 


t» r 


Published for the Association by 
T. A. Randall & Co., Indianapolis, Ind. 














^LIBRARY ofCO N in RESSi 

Two Oopl«8 Receive 

w 31 1908 

vu^ng4K entry 
f-e^L ( r^»i 8 
SCLASSA XXc. f*u. 

(('i 8 & s~S~ 

COPY B. 


Copyrighted 1908 
By T. A. RANDALL, Secy. 


M 












CONTENTS 


• .}, PAGE 

Letter of Transmittal .. 7 

Preface . 9 

Historical Review of Subject. 11 

Work of Seger ........ 12 

Work of Orton and Griffin . 12 

• ' ' i 

Work of Matson . 18 

Bibliography . 20 

Beginning of the Investigation . 21 

Production of Test-pieces .,•. 22 

The 'Clay... !!.... . 22 

The .Burning . 24 

Test-pieces representing Burn A. 29 

Test-pieces representing Burn B. 32 

Test-pieces representing Burn C... 34 

Chemical Work . 38 

Sampling the Draw-trials . 38 

• 

Carbon .;. 52 

Method of Analysis . 52 

Tabulated Results of Analysis . 54 

Rate of Loss of Carbon . 55 

Relation between size and density of coloration of black cores. . 56 

Negative evidence on Carbon as cause of color. 56 

Disappearance of carbon from seemingly vitrified bodies, with¬ 
out causing swelling. 57 

Deposition of carbon by heavy reduction . 59 

Summary of the conclusions on Cakbon. 59 

I ron . 60 

Method oif Analysis—Total Iron . 60 

Ferrous Oxide . 61 

Ferric Oxide . 62 

Tabulated Results of Analysis . 63 

Discussion of the Determinations—Ttoal Iron . 64 


3 

































4 


THE N. B. M. A. COMMITTEE 


PAGE 

Ferrous Oxide and the Raw Clay. 65 

Distribution of Oxides in the Red Portion of the Clay. 66 

Distribution of Oxides in the Black Portion of the Clay. 68 

The Role Played by Iron . 71 

The Black-Coring Reaction . 72 

The Blue-Stoning Reaction . 75 

The Over-Fire Reaction . 76 

Summary of the Conclusions on the Action of Iron. 79 

Sulphur . 80 

Methods of Analysis—Total Sulphur . 80 

Soluble Sulphur .Salts . 83 

Insoluble Sulphur Combinations . 84 

Tabulated Results of Analysis. 85 

Discussion of the Determinations . 86 

Distribution of Sulphur in the Red Portion of the Clay. 86 

Distribution of Sulphur in the Black Portion of the Clay. 89 

Reactions by which Sulphur is Expelled . 92 

The Normal Breaking-down of Pyrites . 92 

The Slow Oxidation of FeS . 92 

The Dissociation of Sulphates . 93 

The Influence of Carbon and Iron. 94 

The Influence of Silicic Acid . 97 

The Residual-G>as Theory of Swelling.T01 

Summary f the Conclusions on the Action of Sulphur.102 


General Conclusions 


104 



























ILLUSTRATIONS 




PAGE 

Fig. 1—Chart Summarizing Conditions of the three Burns. 25 

Fig. 2—Time-Temperature Curve Sheet for Burns used in prepar¬ 
ing Samples . 28 

Fig. 3—Photograph of Samples illustrating Burn A.... . 30 

Fig. 4—Photograph of Samples illustrating Burn B. 33 

Fig. i5—Photograph of Samples illustrating Burn C. 36 

Fig. 6—Photograph of Draw-trial No. 1 . 39 

Fig. 7—Photograph of Draw-trial ONo. 4 . 40 

Fig. 8—Photograph of Draw-trial No. 7 . 41 

Fig. 9—Photograph of Draw-trial No. 8 . 42 

Fig. 10—Photograph of Draw-trial No. 9 . 43 

Fig. 11—Photograph of Draw-trial No. 14 . 44 

Fig. 12—Photograph of Draw-trial No. 21 . 45 

Fig. 13—Photograph of Draw-trial No. 25 . 46 

Fig. 14—Photograph of Draw-trial No. 28 . 47 

Fig. 15—Photograph of Draw-trial No. 30 . 48 

Fig. 16—Photograph of Draw-trial No. 37 . 49 

Fig. 17—Photograph of Draw-trial No. 39 . 50 

Fig. 18—Photograph of Draw-trial No. 40 . 51 

Fig. 19—Sketch illustrating the Relation between Appearance of 

Brickettes and their Carbon Contents. 54 

Fig. 20—Curve-sheet showing percentage of initial carbon remaining 
in the black cores of brickettes after various periods 
of exposure . 55 

Fig. 21—Curve-sheet showing percentage of initial carbon remaining 

after various temperatures . 56 

Fig. 22—Curve-sheet showing distribution of Iron Oxides in the Red 

Portion of the Clay.:. 67 

Fig. 23—Curve-sheet showing distribution of Iron Oxides in the 

Black Portion of the Clay. 70 

Fig. 24—Sketch of an Alcohol Lamp for Sulphur Fusions. 82 

Fig. 25—Curve-sheet showing distribution of Sulphur in the Red 

Portion of the Clay . 88 

Fig. 26—Curve-sheet showing distribution of Sulphur in the Black 

Portion of the Clay. 91 

Fig. 27—Curve-sheet showing Influence of Carbon and Iron on the 

Rate of Expulsion of the Sulphur. 95 


5 































LETTER OF TRANSMITTAL. 


Theodore A. Randall, Esq., 

Secretary National Brick Manufacturers’ Association. 

Dear Sir: — 

It gives me pleasure to submit to you herewith the results 
of the investigation conducted by Mr. Homer H. Staley, B. Sc., 
and myself, on the status of the carbon, iron and sulphur com¬ 
pounds found in clay wares. The study is a chemical one, and 
is designed to shed light upon the behavior of the common, and 
in fact almost universally present impurities of clays, upon 
which we have at present very little direct evidence, and upon 
which much theorizing has been done with but little basis of facts 
for support. 

In this investigation into the intimate chemical causes of the 
familiar phenomena which have been heretofore observed and 
described, some results have been obtained which are of im¬ 
portance, in that they show that the theories thus far advanced 
are in some respects true and in others entirely wide of the mark. 
While the work is by no means complete, it constitutes at least 
a clear step forward. 

Very respectfully submitted, 

Edward Orton, Jr., 

Chairman of the Committee on Technical Investigation. 
Ohio State University, 

Columbus, Ohio, 

January, 1908. 





PREFACE. 


This volume is the product of the seventh year’s work of 
the scholarship provided by the National Brick Manufacturers’ 
Association in the Ohio State University. It was undertaken in 
September, 1904, and finished in July, 1905, except as to pre¬ 
paration for publication, which for various causes has been 
delayed until the present time. 

The work undertaken is the direct continuation of or out¬ 
growth from the work done in the preceding year, 1903-04, and 
which was published as the Second Report of the Committee on 
Technical Investigation, in 1905. This report dealt with the 
behavior of carbon as an ingredient in clays, and developed by a 
long series of synthetic mixtures and by various heat treatments 
of natural clays, a fairly complete demonstration of the behavior 
of carbon in clays and its effects on the iron during the burning 
process. The work did not, however, discover the intimate 
chemical cause of these various characteristic behaviors of clays 
impregnated with carbon. It developed the relationship of 
carbon to the various physical results or defects of the burning 
process, but did not disclose the train of chemical reactions which 
were responsible therefor. 

But, like most other investigations, this one led directly 
to others. There seemed clear indications that sulphur was 
entitled to be considered jointly with carbon as the cause of some 
of these phenomena, and that the precise mode of operation of 
both on the iron of the clay during vitrification and fusion was 
yet to be worked out. The work was undertaken by Mr. Homer 
H. Staley, the scholarship appointee for that year, with great 
vigor and interest, and was carried forward during the year 
without more than general supervision and mapping out of the 
field of investigation. Mr. Staley deserves much credit for the 
way in which he handled the problem, and reduced his data. 

This work has by no means completed what needs to be done 
along this phase of the subject, for it is simply a study of what 
happens in one clay during its various stages of proper and im¬ 
proper firing. 


9 



10 


PREFACE. 


There is need of much other similar work being undertaken, 
because it may well be doubted whether the particular mineral 
aggregation of the clay studied is sufficiently typical to prove the 
case for other clays of different age and geological history. In 
particular, such a research ought to be undertaken on two or 
three synthetic mixtures, in which the composition could be 
controlled by using the various inert and active clay forming 
minerals in different proportions. It is hoped that other ob¬ 
servers may undertake this problem, as this committee has now 
given as much attention to it as can properly be alotted to it from 
among the many other interests pressing for consideration. 

Edward Orton, Jr., E. M., 

Chairman of the Committee . 


A STUDY OF THE CHEMICAL STATUS OF THE CAR¬ 
BON, IRON AND SULPHUR IN CLAYS DURING 
THE VARIOUS STAGES OF BURNING. 

BY 

Edward Orton, Jr., E. M., and Homer H. Staley, B. Sc. 

HISTORICAL. 

In the preceding’ Bulletin of this series* the influence of 
carbon upon the burning behavior of clays was exploited with 
some care, and a fairly successful effort was made to learn the 
order of events under the various proper and improper methods 
of conducting the burning process. These sequences of cause 
and effect were investigated in their physical aspect chiefly, and 
while certain chemical hypotheses were necessarily used in pic¬ 
turing what was taking place in a burning clay, the investigation 
did not proceed rigorously to the proof that the result attained 
was really due to the postulated cause. 

When the physical result attained was in agreement with 
what the chemical hypotheses led us to expect, it was accepted 
as evidence still further strengthening the hypothesis in question, 
though it might be really due to a concurrent cause which had 
not been clearly differentiated. 

Seger early did important work in this field as in most other 
phases of ceramic technology. In a number of papers f, he showed 
that both ferric and ferrous iron coexisted in most brick; that 
the ferrous oxide was never wholly oxidized into ferric, even after 
long continued treatment with hot air; that a considerable quan¬ 
tity of ferrous oxide may not appreciably affect the color of the 
clay, if the ferric oxide largely predominates; that if the ferrous 
oxide becomes as great or nearly as great as the ferric, the color 
is modified, and that ferrous colors are green, gray, black, or blue, 

♦The Influence of Carbon in the Burning of Claywares. Second 
Report on Committee on Technical Investigation. By Edward Orton, 
Jr., E. M. and Carl H. Griffin. Pub. T. A. Randall & Co., Indianapolis, 
Ind., 1905. 68 pp. 

fSee Bibliography following. 


11 



12 


THE N. B. M. A. COMMITTEE 


according to the clay, its density, its stage of deoxidation; that 
complete deoxidation to ferrous iron, with no ferric, is as rare as 
the reverse case, and many other points. 

While a study of Seger’s writings shows that he has con¬ 
sidered deeply and presented data upon almost every phase of 
the status of iron in clays, there still remains uncertainty con¬ 
cerning a number of very important points. Among these, the 
two following may be mentioned: 

(a) Does a red burning clay in undergoing fusion neces¬ 
sarily suffer a change of the ferric to ferrous state, i. e., is the 
darkening in color which generally accompanies fusion necessarily 
a sign of the formation of ferrous oxide ? 

(b) Does ferrous oxide really reduce fusion temperatures 
or not, i. e., is. a ferrous silicate per se more fusible than a ferric 
silicate of the same constitution in other respects ? 

An effort to get at answers to these points was made in the 
conclusion of the above mentioned report (No. 2). The evidence, 
though not conclusive, has some value, and for the sake of con¬ 
tinuity between the preceding year’s work and the present, this 
data is here reproduced in full: 

“Upon the completion of the experimental work to deter¬ 
mine the influence of various factors upon the rate of carbon 
expulsion, a series of chemical analyses were engaged in to 
verify the conditions of the iron which have been assumed as 
existing in the normal and in the black-cored portions of clay- 
wares. Ten vitrified commercial clay products, all showing 
this phenomenon to a more or less marked degree, were 
selected for this investigation. From each of these, two sam¬ 
ples were prepared, the one designated as A in the table of 
analyses, from the exterior portion of the ware, the other, 
marked B, from the interior. 

Samples.—A description of the products from which these 
samples were taken, and the extent to which they have been 
affected by this reaction, is given as follows: 

1. A stiff mud brick made from a low grade fireclay and 
burned in a test kiln at the Ohio State University. This 
samples hows a typical black core reaction, being composed 
of a comparatively thin band of pinkish color, surrounding a 
black core in which the structure has become more or less 
vesicular. 

2. A dry press face brick composed of a No. 2 fireclay 
containing one-half per cent, of concretionary iron in the 
ferrous condition, manufactured at Canandaigua, N. Y. This 
sample is flashed on the exterior, showing numerous black 


ON TECHNICAL INVESTIGATION. 


13 


spots of iron slag upon a field of a warm brown color. The 
interior is light grey with a black center in which no swelling 
is evident. 

3. A piece of drain tile from a plant near Ottawa, Ohio, 
made from an ailuvial clay of a thoroughly oxidized character. 
Notwithstanding the fact that the iron in this clay was origi¬ 
nally in the ferric condition, this specimen shows as good an 
illustration of the black-coring reaction could be found. The 
exterior of the tile is of a fine red color, penetrating to a depth 
of one-eighth of an inch, while the interior shows a black band 
three-fourths of an inch in width on a cross-section one inch in 
diameter. In ordinary practice, this clay would probably give 
no indication of this reaction, the trouble in this case having 
arisen from an unusually short firing period (twenty-four hours 
from lighting kiln to bright redheat). 

4. A piece of Wassail block, made from an ordinary shale 
of Devonian age, occurring near Glouster, Ohio. This sample 
shows an extremely thin reddish film upon the exterior, the 
interior color being black, with a dark red core, showing that 
brick had once been perfectly oxidized. This brick has evi¬ 
dently been subjected ’to a heavy reduction, at the close of the 
burn, which in some places has penetrated to a depth of one 
inch, the surface color being due to reoxidation in cooling. 

■5. A dry press test block, made by W. D. Richardson, of 
Columbus, Ohio, and burned in a small up draft gas kiln. The 
clay used by him in this experiment was prepared by passing 
through fine screens, thus giving a finished brick of an ex¬ 
ceedingly dense structure. This specimen shows a good red 
color, showing that it has either been reduced from a pre¬ 
viously ferric condition or has failed to oxidize by contact of 
flame. This is not a typical black-core reaction. 

6. A test piece of Haydenville fireclay, prepared in a wet 
way by passing through a 150-mesh screen, showing a thin 
oxidized film of a greenish grey cast, surrounding a bluish-black 
interior in which swelling has begun. 

7. A piece of ordinary shale sewer pipe from North 
Columbus, Ohio. This sample shows a typical case of black¬ 
coring, the unoxidized portion, however, not being of sufficient 
size to cause swelling. 

8. A red-burning paving brick showing practically the 
same phenomenon as sample No. 4. The brick has at one time 
been thoroughly oxidized, but the color of the exterior has been 
changed to a greenish-black by a subsequent reduction. 

9. An ordinary shale paving block which has reached 
vitrification without having first been thoroughly oxidized. 
The exterior of this brick is of a normal red color, while the 
interior is composed of a mass of spongy black slag. 

10—A. A dry press floor tile from the plant of the Mosaic 


14 


THE N. B. M. A. COMMITTEE 


Tile Company, of Zanesville, Ohio. This sample is hard-burnt, 
being practically non-absorbent, and has a fine red color, which 
indicates perfect oxidation. 

10—B. A piece of similar tile, composed of the same 
material. This sample had been fired to a higher temperature 
than the preceding one, but shows the same fine red color on 
the .surface. The interior, however, is evidently suffering from 
the bluestoning reaction, as is shown by the appearance of 
mottled-black colorations irregularly disposed through the 
mass. 

10—C. This sample represents the same material over¬ 
fired. It shows a thin film of a greyish-brown caste on the 
surface, the interior being of a bluish-black color, and showing 
an extreme case of bluestoning. No red color is left. 

The first point, however, which seems to demand explana¬ 
tion is, whether any carbon, as such, exists in the clay after 
vitrification, or whether the black coloration, known as black 
core, is due entirely to the presence of iron as the ferrous 
oxide. In the Thonindustrie Zeitung* the statement is made 
that black colorations in the interior of clay wares are due to 
ferrous oxide, derived from ferric oxide through the agency of 
organic matter present in the clay. Hopwood and Jackson, 
of the English Ceramic Society, state that this same phenome¬ 
non is due principally to the presence of free carbonaceous 
matter in the clay, which, by reason of an insufficient air 
supply, has been given no opportunity for oxidation, but that 
the effect of this agent is modified to a greater or less extent 
by ferrous oxide, ferroso-ferric oxide, and occasionally by 
sulphide of iron.f 

Carbon Determination.—In the expectation of finding that 
the black color in the affected parts of these samples was due, 
in part at least, to the presence of free carbon, analyses for 
this substance were made in the laboratory of Prof. N. W. Lord, 
of the Ohio State University. These determinations were made 
by previously boiling the powdered samples in H'Cl to expel 
C0 2 , then igniting to a good red heat in an atmosphere of pure 
O, and determining the amount of CO., evolved, gravimetrically, 
by absorption in a weighed amount of caustic potash. 

In none of these samples was any important amount of 
carbon found, the highest percentage shown in any case being 
well within the possible limits of error of the analysis. . It is, 
of course, possible that had the silicate structure of samples 
been unlocked by digesting with hydrofluoric acid previous to 
their ignition, some carbon might have been found, but this is 
only a remote possibility, suggested by the results obtained by 


*No. 90—Page 1,432, 1903. 

fTrans. Eng. Cer. S., Vol. II, Pages 100-107. 



ON TECHNICAL INVESTIGATION. 


16 


Hopwood and Jackson, the truth of which can only be demon¬ 
strated by further experiment. 

The samples analyzed by Professor Lord showed after 
ignition a decided transition in color from the bluish black to 
a red-brown tint, which, in the absence of carbon, indicates that 
the oxidation of the iron from the ferrous to the ferric condi¬ 
tion has been responsible for the change. 

Probable Condition of the Carbon. —It is probable that the 
carbon still remaining in the clay at the beginning of vitrifica¬ 
tion is converted into carbon monoxide or carbon dioxide by a 
reduction of the iron, and is retained in the mass of the clay 
under pressure. If the temperature is not increased beyond 
that point, these gases will give no indication of their presence, 
but will gradually Alter out through the almost solid mass. 
But, on the other hand, if the heat be raised to a point at 
which the clay begins to soften, the increased pressure of the 
gases would cause the softened mass to swell and expand. 
The gases may or may not escape, depending on the degree of 
fluidity reached. Since the samples had been pulverized and 
boiled with HC1 to expel all gases before ignition, the above 
theory. is a plausible explanation of the absence of carbon 
as shown by these analyses, and whether it will Anally be 
accepted or rejected, will depend upon future work to be 
carried out along the same general lines. 

Condition of the Iron. —The next point to be considered 
was the condition of the iron in the normal and blackened 
portions of these same samples, determinations being made of 
the percentage of ferric and ferrous iron in each. The results 
of these analyses have been set down in the accompanying 
table. 


16 


THE N. B. M. A. COMMITTEE 


Table No. 1. 


Sample No. 

Percent FeO. 

n 

c 

o 

<u 

■+-* 

C 

<u 

o 

<D 

X 

n 

O Qj 

►—1 ^ 

U1 

4-» 

o 

H 

CHECK DETERMINATIONS. 

On FeO. 

On 

Total Iron 
as Fe 2 0 3 . 

1 

2 

3 

1 

2 

3 

1-A. 

0 95 

4 95 


0 86 

0.90 

0.95 

5.95 



1-B 

5.27 

0 15 

6.00 

5.18 

5.27 


6.00 



2-A. 

1 13 

4 15 


1.13 




*5 72 


2-B . 

4.73 

0 20 

5.45 

4.73 

4.73 


5 40 

5 45 


3-A. 

0.45 

6.20 


4 55 

4.55 





3-B 

5.90 

0 16 

6 70 

5.72 

5.81 

5.90 

6.60 

6 70 


4-A. 

4.73 

1.15 


4.68 

4.73 





4-B 

0.86 

5.45 

6.40 

0 90 

0.86 


6 40 



5-A. 

3 74 

2.25 


3.47 

3.74 





5-B. 

0.77 

5 55 

6.40 

0 77 



6 10 

6 40 


6- A . 

1.49 

2.15 


1.35 

1.49 


3.80 



6-B 

3 02 

0.45 

3.80 

2.88 

2 84 

3 02 

3 80 

*3.71 


7-A 

1.04 

6.35 


1.04 

1.04 





7-B . 

5.45 

1 45 

7.50 

5.45 



7 50 



8-A . 

4 28 

2.95 


3.92 

4 28 





8-B .. 

0.81 

6.81 

7.70 

0.81 

0.81 


7 70 



9-A. 

0.63 

6 51 


0.63 

0.63 





9-B . 

5.63 

0.95 

7.20 

5 54 

5.63 


7.20 



10-A 

0.77 

6 45 

7.30 

0 77 



6.40 

7.30 


10-B 

1.22 

5.95 

7.30 

1.22 

1.22 


7.30 



10-0 .. 

2.12 

5.05 

7 40 

1.85 

2.12 


7.60 

7.40 

7.40 


*By the K 2 Cr 2 C >7 method, 






























































































ON TECHNICAL INVESTIGATION. 


17 


In general the above results show that the condition of the 
iron existing in the black, discolored portions of these samples 
is chiefly ferrous, and that in the red, normal portions, the 
ferric predominates. A most notable exception to this rule 
occurs in the case of Sample 10 j C, which, it will be remem¬ 
bered, is a dry-press floor tile, suffering from an exaggerated 
case of bluestoning, and which upon fracture shows black from 
surface to surface. Judging by the results obtained in the 
preceding analyses we would expect to find that the iron was 
largely in the ferrous or black condition, and that the percen¬ 
tage of ferric iron was correspondingly small. As a matter of 
fact, however, the results show that just the reverse is true, 
and that the proportion of ferric iron to ferrous is approxi¬ 
mately as 2.4 to 1. 

It will be noticed that in those cases in which the black 
colorations have been formed on the exterior of the ware by 
the flashing or reduction oif a previously oxidized portion, as in 
samples 4, 5, and 8, the percentage of ferric iron is relatively 
higher than in those formed by a typical black-core reaction. 
Sample 10-C is closely associated with this latter group of 
phenomena, in the origin of its present condition, which has 
been brought about by the reduction of a previously oxidized 
body. This, however, is as far as the relation extends, as the 
agents by which the reduction was produced are very widely 
different. In this case, the tile was subjected to a temperature 
too high to develop its best properties, sufficient of the iron 
thus being reduced to mask the red color of the ferric oxide and 
give to the whole mass a bluish J black appearance. This phe¬ 
nomenon represents the first step in the process of fusion, and 
it is probable that if the heat had been maintained at the 
highest temperature reached in the burning of this tile, for a 
lengh of time sufficient to have completely reduced the iron, 
this reaction would have taken place. 

The result of these analyses has been to show that in the 
black, discolored portions of the samples, the iron is largely in 
the ferrous condition, while in the red, normal portions, the 
iron occurs principally as the ferric oxide. They also indicate 
that the coloring power of ferrous oxide is in itself sufficient in 
intensity to produce dark colorations, even in the entire ab¬ 
sence of carbon, and that while they have not proven con¬ 
clusively that there is no carbon present in these black cores 
after the clay has been vitrified, the evidence strongly favors 
such a conclusion.” 

Other evidence bearing on this same question was published 
by Mr. George C. Matson in the Clay worker for July, 1904, as a 
result of work done under Dr. IT. Ries, at Cornell University. 


18 


THE N. B. M. A. COMMITTEE 


“At the suggestion of Professor H. Ries, the writer under¬ 
took a series of experiments to determine whether the iron in 
clays returned to the ferrous condition as the clay was burned 
to viscosity in an oxidizing atmosphere. The results are given 
in the accompanying Table of Analysis. (See opposite page.) 

“The clays were first molded into small bricks and then 
burned in a gas furnace, care being taken to raise the tem¬ 
perature slowly, and to keep the atmosphere of the furnace 
oxidizing. As soon as the burning was finished, the burned 
clay was covered with powdered feldspar or kaolin to prevent 
oxidation of the iron while cooling. The material for analysis 
was taken from the outside .of the brick. 

‘ The increase in the percentage of iron in the burned clay 
was due to the expulsion of water and otheir volatile substances 
in burning. All the analyses showed a decrease in the percen¬ 
tage of ferrous iron, and this taken in connection with the 
increase in the percentage of the total iron shows that the 
ferrous iron tends to change to ferric iron, even w'hen the clay 
becomes viscous. However, with one exception (the clay from 
Amenta, N. Y.) there was ferrous iron present, which would 
indicate that the oxidation of the iron in the interior of the 
bricks was not complete; probably because the oxygen of the 
air did not penetrate to the interior very readily. Nevertheless, 
the results show that some oxidation took place even there. 
From the analyses, it would appear that the iron does not re¬ 
turn to the ferrous condition except when burned in a reducing 
atmosphere. 

“While this rule holds true for ordinary clays, it might not 
be true for a clay containing a large percentage of carbona¬ 
ceous matter, which would tend to reduce the iron to a low 
state of oxidation unless the temperature was raised slowly 
enough to drive off the carbonaceous matter. 'Moreover, in the 
case of clays burned at a very high temperature, it may be 
impossible to maintain an oxidizing atmosphere in the furnace; 
and hence the iron will naturally be reduced.” 


The evidence of this work from various sources shows 


1st. That carbon is ascribed as the cause of the black color¬ 
ations in improperly fired clay wares, but that the evidence so 
far available throws much doubt on this source as an important 
cause. 


2nd. That the presence of iron in the ferrous condition is 
ascribed as the cause of black colorations in the center of either 
improperly burnt clay wares, or on the exterior of clay wares 
which are either reduced by smoky firing at high temperatures, 
or even by mere over-heating in oxidizing atmospheres. The 


Table of Analysis. 


ON TECHNICAL INVESTIGATION 


19 


1 

G 















_ G 















2= 3.2 

o 

GO 

o 

o 

o 

o 

O 

O 

o 



O 

o 

00 

o ~ o 

CO 

1—1 

05 

05 

H 

GO 

o 

05 

CO 



CO 

to 


ft 

<M 

CO 

ft 

GO 

00 

05 

• 

00 



to 

CO 


u sJ 

CO 


r “" 

LO 


Cl 

CM 

rH 




CO 


t—( 















U 
















t— 

t—H 

y—t 

GO 

O 

r-H 

CO 

05 

o 

05 

to 

CO 

to 



rH 

rH 

O 

O 

tO 

CM 

r- 

*—< 

05 

CO 

oo 

CO 

CO 



GO 

L" 

rH 

CO 

CO 

(M 

05 

Ol 

05 

rH 

rH 

r ”! 

o 



ft 

05 

05 

<M 

ft 

ft 

co’ 

05 

rH 

CO 

H 1 

CM 

to 











r 



CM 

'T 


in 

G 

o _ 

o 

CO 

05 

T'- 

o 

05 

00 

lO 

o 



rH 

o 


u a 

LO 

co 

I- 

05 

05 

05 

05 

r 

05 




ft 


»- o 

0) U 

tO 

CM 


CO 

rH 


CO 

CM 

o 



M 

co 


{jj rH 

CM 

r-H 

rH 

cm 

rH 

rn 

CM 

rH 

rH 



OJ 

rH 


















I- 

o 

ft 

tO 

o 

o 

rH 

HH 

o 

05 

tO 

■^i 

05 


- a 

o 

05 

co 

o 

ft 

CM 

r- 

CO 

CO 

CO 

00 

o 

L— 


r O 

CO 


co 

05 

tO 

O 

co 

rft 

o 

rr 

rH 

T 

co 




o' 

o 

ft 

CO 

CO 

GO 

O 

co 

co 


—H 

co 



rH 

rH 





rH 




co 

ft 




o 

o 


o 

o 


o 

o 


O 


o 


d 


05 

05 


05 

05 


05 

CO 


co 


CO 



©1 

rH 


CM 

rH 


rH 

CM 


CM 


co 




rH 

*-H 


r —h 

rr 


f-H 

r-H 


rH 


i-H 


Cl hl. 


1 

1 


l 

i 


1 

i 


1 


i 

Tf 

g 1 


ft 

ft 


ft 

ft 


~P 

co 


co 


co 




to 

I- 

I 

tO 

t— 



Hi 


Hi 


(M 




co 

rH 

/ . 

co 

rH 


rH 

M 


©1 


ft 


“ G 

O 


M 

CM 


(M 

cM 


CM 

Ol 


M 


(M 


CJ 


GO 

CO 


1 

QO 

CO 


CO 

lO> 


io 


O 















rH 




Xfl 

Xfl 



a 












a 



a 












a 






Xfl 



Xfl 






H 



o 



a 



a 


Xfl 


u 

3 

■*-» 

x 


pH 

eg 

p—H 
£ 

o 

Xfl 

0 


•rH 

co 

O 

i> 

co 

a 

o 

a 


o 

fH 

o 

ft 

a 

o 

pH 

o 


o 

Ph 

O 

ft 


a 

o 

Ph 

o 

ft 

>> 

CO 

OJ 


• iH 

go 

© 

o 

pH 

o 


k co 
■0 «! 
o a 

o 

ft 


4-h 

ft 

Ph 


4-h 


'd 


i> 

A 


ft a 

£ 

o 


H 


rH 


Ph 


G 

G 


-o’ 

© 

' o 
<0 

Ph 


rH 


be 

•rH 

© 


be 

• fH 


<D 

> 


jd 


Ph 

O 


o 

-p 

"© 


xn 

TO 


GO 


M 

o 


u 


4ft 



tO 

© 

nn 



-6 

© 

Ph 

© 

Ph 


TO 

© 

Ph 


_a 

M 




<—H 



© 












0> 



a 










G 



to 


to 



tO 

'S 


tO 


TO 


O 

£ 

© 

<0 

£ 

© 

© 


© 


© 


© 

N 

*3 

G 

cS 

Ph 

a 

H 

a 

a 

Ph 

a 

c3 

P3 

|H 

H 

a 

rH 

M 

a 

Ph 

a 

a 

a 

rH 

PH 

a 

(M 

Ph 

Ph 

a 

a 

Ph 

a 

Ph 

a 


o 


PQ 

PQ 


PQ 

PQ 


rH 

PQ 


PQ 


PQ 














Ph 



















r* 











ft 

CC 




. 











TO 















*© 















[ft 



* 

bjj 

<r 

» 










TO 

• 

*—< 

— 

• pH 

* 


. 

ft 






rH 

•H 



o 

TO 

cH 

H 



£ 








03 

© 




c3 











4-^ 




ft 



1 O 

i —H 






a 


• f—H 

4ft 







05 






• rH 


© 




rH 



ca 

V* 


V 


V 

HH 


(H 




4ft 

• rH 
cH 

rj 

C/2 


-* 

£ 

0) 

fc 






© 

rH 

a 


be 

c3 

% 



In column 1, the name applies to the locality from which the clay was obtained. In column 4, the tempera¬ 
ture, Seger’s cones were used. The F. and C. equivalents are from the Report on the Clays of Michigan, by H. Ries. 
In columns 5, 6 and 7, the per cent, is based upon the total weight of the clay; in column 8, the per cent, is based 
upon the total iron. 
























































20 


THE N. B. M A. COMMITTEE 


evidence so far collected favors this view as the common and 
potent cause of black colorations. 

3rd. That iron in the ferric condition may exist in predomi¬ 
nating amount intermixed with that in the ferrous condition, 
without imparting a red or brown color to the clay ware. 

4th. That sulphur exerts an influence in some ways com¬ 
parable to that of carbon, and that its behavior, rates of oxida¬ 
tion, and effect on color, vesicular structure, tec., have yet to be 
demonstrated. 


BIBLIOGRAPHY. 


Articles in the Collected Writings of Herman A. Seger. 

(1) Some Investigations Pertaining to the Colors of Bricks 
Page 106. 

(2) The Influence of Fire Gases upon Clays and the Color 
Phenomena Connected with It. Page 123. 

(3) Studies in Regard to the Composition and Action of 
the Fire Gases in the Kilns of the Ceramic Industry. Page 135. 

(4) Notes on Brick Colors. Page 343. 

(5) The Natural Colors and Discolorations of Light- 
Colored Bricks. Page 347. 

(6) The Colors of Bricks. Page 360. 

(7) The Influence of Sulphur in Coal upon Clay Wares. 

(8) The Influence of Sulphuric Acid on Glazes and Bodies. 

(9) Phenomena and Precautions to be Observed in the 
Burning of Calcareous Clays. Page 971. 

(10) The Coloration of Clay wares by Iron at High Tem¬ 
peratures. Page 1027. 

(11) Coloration of Porcelain in the Glost Burn. Page 1041. 


Articles in the 


Transactions of the American 


Ceramic 


Society : 

(1) On the Role Played by Iron in the Burning of Clays. 
Orton, Yol. Y, p. 377. 

(2) The Blistering of Glazes. Vol. II, p. 139. 

Articles in the Publications of the National Brick Manufac¬ 
turers * Association. 


(1) Formation of Dark Cores or Discolorations in the In¬ 
terior of Clay wares. Official Report 1904, page 88. 

(2) The Influence of Carbon in the Burning of Clay wares. 


ON TECHNICAL INVESTIGATION. 


21 


Second Report of the Committee on Technical Investigation. 
Indianapolis, 1904. 68 p. 


Articles in the Transactions of the English 
Society : 


Ceramic 


(1) The Colouration of Clay wares, by A. Hop wood and 
W. Jackson, Transactions of the North Staffordshire Ceramic 
Society, 1901-02, page 92. 

(2) The Changes in Colour of Clays on Ignition in Clay- 
ware Kilns, by Arthur Hopwood, Transactions of the English 
Ceramic Society, 1903-04, page 37. 


Miscellaneous articles : 


The Behavior of Iron in Clay when Burned to Viscosity, by 
Geo. C. Matson. Clay worker, July, 1904. Pub. Indianapolis, 
Indiana. 

Reports on clay in the various state geological survey 
reports. 


II. THE INVESTIGATION 


Toward the settlement of the actual status of these three chief 
impurities of clays, during the various stages of burning, the 
following work was undertaken: 

1st. To produce a carefully graduated series of samples, all 
from one homogeneous batch of the same clay used in the pre¬ 
ceding studies, viz., the Black or Huron Shale of Columbus, 
representing all stages of the process of proper and improper 
firing. 

2nd. To determine by careful analyses the quantity and 
chemical condition of each of the three substances, carbon, sul¬ 
phur and iron in each sample. 

3rd. To discover by study of the results what the behavior 
of these three substances is in this clay during firing, and to make 
such deductions as to their probable influence in other clays as 
the data seems to justify. 

The selection of the Huron Shale was made for several 
reasons: 

1st. It is known to be among the worst clays which has been 
commercially used for any length of time as the basis of clay 
manufacture. It has supported clay industries for a period of 
fifteen or twenty years in one locality and less periods elsewhere. 


22 


THE N. B. M. A. COMMITTEE 


but in all places it is recognized as being on the very border line 
of unsuitability, and its use is now at a lower ebb than at any 
time in forty years in central Ohio. 

2nd. It contains all three elements whose behavior is to be 
studied, in considerable amounts, and in such natural forms as 
are common in other carbonaceous or sulphury clays, thus saving 
the need of preparing synthetic samples. 

3rd. The supply was here cheap and easily renewable, and 
oouTd therefore be studied at minimum expense. 

The work of the investigations naturally falls into two 
divisions, as follows: 

IIr. Producing the test-pieces, which should illustrate the 
desired range of chemical and physical conditions. 

IV. Analyzing these test pieces for the amounts of the 
carbon, ferrous oxide, ferric oxide, total iron, sulphur existing 
as soluble sulphates and insoluble silicate or sulphide forms, and 
total sulphur, and drawing the conclusions from the data. These 
divisions will now be taken up in turn. 

Ill* PRODUCING THE TEST-PIECES. 

The Clay. The North Columbus Shale is thus described in 
an earlier report, in which it was used: 

“This is a typical shale of the carbonaceous variety which 
is obtained from an outcrop of Devonian age at North 
Columbus, Ohio. The face of the exposure which is being 
worked there is from fifteen to twenty feet in depth and shows 
alternating strata of black and blue. The black layers are 
generally thinner than the blue, comprising approximately one- 
third of the deposit, and owe their dark color to the presence 
of organic matter as albertite or pitch. This black portion is a 
hard bituminous shale, with low plasticity and high carbon 
content, while the blue variety is comparatively soft, lower in 
carton, and readily slakes down to a plastic clay upon exposure. 
This wide variation in physical properties is not due to the 
presence of carbon per se, but to the asphaltic character in 
which it occurs, which is supposed to have originated from a 
slow destructive distillation of the spores of coniferous plants, 
deposited in the clay at the time of its formation. This shale 
contains 5 to 7 per cent, or iron, present chiefly as the car¬ 
bonate, and occasional marcasite concretions, but there is also 
a small amount of ferric iron as is shown by analysis. Another 
factor with which the brickmaker has to contend is the pres¬ 
ence of gypsum or sulphate of lime, which causes the brick to 


ON TECHNICAL, INVESTIGATION. 


23 


scum badly in the burning. Anywhere along the face of the 
clay pit, where the shale has not been worked for some time, a 
white efflorescence of this salt is found. 

Two analyses of this shale, made at different times and on 
different samples at the Ohio State University, are given below: 


TABLE No. 2. 


CONSTITUENTS 

l* 

2t 

Silica . 

58.38 

57.69 

Alumina . 

20.89 

20.05 

Ferrous Oxide, FeO. 


7.07 

Ferric Oxide, Fe.O. 

6.78 

1.08 

Lime . 

0.44 

0.61 

(Magnesia . 

1.57 

1.23 

Potash . 

4.68 

3.94 

Soda . 

Loss on ignition, including 

0.34 

0.43 

Water, Carbon and C0 2 . . 

7.53 

8.24 

Total. 

99.61 

100.34 


It will be noticed that in analysis No. 1, the iron is given 
as the ferric oxide, no ferrous iron being noted. The ordinary 
practice in analyzing clays has been to determine the total iron 
as Fe 2 O a , no account being taken of the condition of the iron in 
the original sample, hence the above result. Now, it is the 
custom among ceramic chemists to determine not only the total 
iron as in former years, but to note the distribution between 
the ferrous and ferric forms. 

It will be noticed that in neither of these analyses was the 
sulphur determined, though of course it was partly included in 
the loss on ignition over the blast lamp. 

Preparation of the trial-pieces. 

The crude clay was ground in a small dry-pan and screened. 
The screenings were further ground in a large mortar until all 
had passed through a seive of 16 meshes to the linear inch. The 
mass was then tempered in a pugmill with distilled water, up to 
the consistency required for stiff-mud bricks, and was then 
blanketed and allowed to stand for several days, in order that 

♦Professor William 'McPherson, Analyst. 

|Professor Albert V. Bleininger, Analyst. 























24 


THE N. B. M. A. COMMITTEE 


the water might become uniformly distributed and all hard 
grains have opportunity to soften if they would. Distilled water 
was used so that no iron salts or sulphates might be introduced 
into the clay with the water, as the ordinary hydrant water in 
Columbus contains noticeable amounts of both. 

The tempered clay was finally wedged by hand, on a clean 
board, to expel inclosed air and obtain the best homogeneity. 
The use of a regular plaster wedging block was avoided for fear 
of the introduction of particles of CaS0 4 . The samples were 
pressed in a hand power screw tile press, in which a dense uni¬ 
form structure can be obtained without the structural defects 
ordinarily found in bricks where made by causing clay to How 
through a die under pressure. 

The size of the brickettes was 4 1 4x4 1 / 4x2 l /5 in the green 
condition. They shrank moderately in drying, being about 
4x4x2 5-16 when dry. These brickettes were about equivalent 
to half brick in size, having the full cross-section of standard 
brick as to breadth and thickness, and only differing from them 
in length. They dried without cracking in the ordinary room 
temperature of the University laboratory. 

The Burning. It was the intent in the burning process to 
produce three sets of brickettes, viz.: 

Burn A. A set which should be fired rapidly through the 
oxidizing period, without affording any adequate time for oxida¬ 
tion, and then carried steadily, but at normal speed through the 
vtrideation period. 

If trials were drawn at stated intervals in such a heat treat¬ 
ment, and cooled rapidly and then broken, they would show a 
series of black-cored products, changing from the soft crumbly 
center bordered by a flesh pink crumbly exterior during oxida¬ 
tion, to a hard dense black center, bordered by hard dense red 
exterior during vitrification, and to a puffy scoriaceous black- 
center during overfire, bordered by a red exterior of dense or at 
least less scoriaceous fracture, according to the degree of overfire. 
This series was designed to show the results of bad or hasty oxi¬ 
dation, good vitrification conditions, followed by overfire. 

Burn B. A set which should be fired slowly until oxidation 
was complete at a low temperature, followed in turn by vitrifica¬ 
tion under good conditions, and finally by overfire. This series 


ON TECHNICAL INVESTIGATION. 


25 


was designed to represent normal or good burning, except it 
was to be allowed to go too far. 

Burn C. A set which should be fired slowly until oxidation 
and vitrification were complete, when a period of heavy reducing 
treatment without increase of temperature would begin, followed 
by raise of temperature sufficient to cause overfire, still main¬ 
taining reducing conditions. 

This series was designed to show the results of proper firing 
during the early portion of the burning, fololwed by improper 
conditions at the finish. This is a very common condition to find 
in burns of every kind of ceramic product. 

These three heat treatments may be graphically illustrated 
by the following diagram : 


Burn A 

Burn B 

Burn C 

Oxidation 

Bad. Too rapid. 

Oxidation 

Good. Slow as 

necessary. 

Oxidation 

Good. Slow as 

necessary. 

Vitrification 

Good. Oxidizing 
Conditions. 

Vitrification 

Good. Oxidizing 
Conditions. 

Vitrification 

Good at first. 
Turning to reduc¬ 
ing towards end. 

Over-Fire 

Good. Oxidizing 
Conditions. 

Over-Fire 

Good. Oxidizing 
Conditions. 

Over-Fire 

Bad. Heavily 
Reducing. 


Figure 1. Comparison of the three heat treatments. 


To attain these three sets of conditions with the least labor 
and delay, two kilns were used simultaneously. Both were small 
test-kilns, but one was larger than the other and will be known as 
the large kiln. The large kiln was a down draft, having but one 
fire box. From the fire box the gases pass up flues on either side 
of the chamber in which the ware is set, enter it through small 
holes in the side walls near the top, pass downwards through the 
ware chamber and out through openings in the floor of the ware 
chamber into a flue which empties into the stack. The ware 
chamber of the kiln measures 20 inches wide x 30 inches long x 
SO inches high. As this space was too large for our purpose, it 

















26 


THE N. B M. A. COMMITTEE 


was filled in at the top and bottom with a checker-work of burnt 
fire-brick. In the center, an open chamber was left among the 
fire-brick, which were placed loosely and without mortar, to 
facilitate the flow of gases through the test brick. With a little 
care in tiring, it was possible to maintain continuously oxidizing 
conditions in this ware chamber. 

The smaller kiln was an up-draft muffle. It has but one 
fire box, which is provided with a coking plate. The muffle is 
directly above the fire, but about two feet distant. It consists 
of a tight brick box, built of well-mortared fire-brick, supported 
on arches. Both sides, one end, and the top and bottom are 
exposed to the play of the flames and combustion gases. The 
other end contains the wicket and is necessarily left unexposed 
to the hot gases to facilitate drawing trials. The inside measure¬ 
ments of the muffle are 10 inches high by 1314 inches wide by 
27 inches long. The interior of this muffle being sealed off from 
the combustion gases, it is possible to maintain quite exact 
control of the composition of the atmosphere in it. 

The plan of the burning was to place all the trial pieces 
in the larger kiln, carry the temperature with reasonable speed 
up to about 800°C, and to hold it there under oxidizing condi¬ 
tions until trials withdrawn should show a sufficient zone of 
oxidized clay on the exterior, surrounding a large black core 
still heavily charged with carbon . When oxidation should have 
reached the desired point about one-third of the samples were 
to be removed and quickly transferred to the smaller or muffle 
kiln, which in the meantime was to be heated up to 800°C. In 
this kiln, the imperfectly oxidized brickottes were to be treated 
according to the program for Burn A. Meanwhile, the larger 
kiln was to be carried along at 800°C under favorable oxidizing 
conditions until the last of the carbon had disappeared from the 
center of the two-thirds of the brickettes still remaining in it. 
This was determined, of course, by drawing and breaking trials, 
until the oxidation was complete. When burn A was completed, 
and the last sample drawn, the muffle was cooled down to 800°C 
and a second lot of samples, now fully oxidized, were quickly 
transferred from the larger kiln to it. These were then treated 
according to the plans for Burn B. When Burn B was in turn 
complete and the trials all drawn, the muffle kiln was to be again 
cooled down to 800°C, and the final third of the brickettes was to 


ON TECHNICAL INVESTIGATION. 


27 


be transferred from the larger kiln, and treated according to the 
plans for Burn C. 

The burning was carried out strictly according to this 
program. Forty samples were placed in the. large down-draft 
kiln. Care was taken to separate them from the floor and each 
other by use of small spacers, set on edge. In this way, all sides 
of each brickette were almost equally exposed to the play of the 
gases. The thermocouple of a he Chatelier pyrometer was placed 
directly in the midst of tin 1 samples, in order to get an exact 
record of the temperature. 

The kiln was heated up with an abundant excess of air, as 
fast as was feasible without injuring the ware. 

The time-temperature curve of both kilns and for all three 
heat treatments is shown in Figure 2: 


28 


THE N 


B. M. A. COMMITTEE 





















































ON TECHNICAL INVESTIGATION. 


29 


The dotted line indicates the progress of events in the 
down-draft muffle kiln, while the solid lines show the progress of 
the several burns after the brickettes had been transferred to 
the up-draft muffle kiln. The gaps between the starting points 
of the curves of Burn B were due to the failure to record obser¬ 
vations promptly after making a transfer of brickettes from the 
first kiln to the second. The gap between the end of the dotted 
line curve, at the 65th hour, and the beginning of curve for 
Burn C, at the 83rd hour, was due to using the thermocouple 
exclusively in Burn B during this period. When the temperature 
of the down-draft kiln was taken, at the end of Burn B, it was 
found that it had crawled up to 890° C, and after transferring 
to the smaller kiln, the temperature was brought back to 810°C 
for a few hours before starting on the reducing burn. The tem¬ 
perature did not go as high in Burn C as in A and B, because 
the reduction caused the brickettes to slag equally at a lower 
temperature. 

As can be seen from Figure 2, trial No 1 was drawn at the 
seventeenth hour of the burn, at a temperature of 710°C. It 
was cooled and broken across the center. This process was re¬ 
peated throughout all the burns, trials being drawn at sufficiently 
short intervals to enable the operators to keep close watch on all 
the changes that were taking place in the ware, and thus secure 
samples at any desired stage of the burning process. 800°C was 
reached at the twenty-first, hour of the burn, and the large kiln 
was then held at approximately that temperature, and under 
thoroughly oxidizing conditions, till the last brickettes had been 
drawn from it at the 83rd hour. 

Trial No. 5, drawn at the thirty-seventh hour of the burn, 
showed that oxidation had proceeded fairly well into the bricks. 
At this point twelve samples were quickly transferred to the 
muffle kiln. Both kilns were at approximately 800°C. These 
constituted the samples for burn A, and the treatment they 
received was according to the program already described for that 
burn. The time temperature curve and indications of the points 
at which trials were drawn is shown. 

Photographs of typical trial pieces from Burn A are shown 
in Figure 3. 


£0 


THE N. B. M. A. COMMITTEE 



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ON TECHNICAL INVESTIGATION. 


31 


The following is a description of the heat pieces shown in each 
onp: 

Draw No. 1, taken at 17 hours after beginning of the burn. 
Temperature 710°C. Exterior oxidized layer one-quarter to 
three-eighths inch thick, pale pink color. Interior unoxidized 
core, dark, almost black, with sharply defined margin. Both 
portions soft and crumbly. 

Draw No. 4, taken at 31 hours. Temperature 805°C. Ex¬ 
terior oxidized portion, one-half to five-eighths inch thick, 
brighter pink color. Interior core, lighter, grayer color and 
shades into exterior zone more gently. Both portions still soft. 

Draw No. 5, taken at 37 hours, at time of transfer of brick- 
ettes from downdraft kiln to updraft muffle for the beginning 
of the special treatment. Temperature 760°C. Physical char¬ 
acteristics unchanged from draw No. 4, except slight decrease 
in size of the core. 

Draw No. 7, taken at 44 hours. Temperature 980°C. Areas 
of outer oxidized zone and inner unoxidized core about, as in 
No. 4, but edges of areas are more sharply defined. Color, 
outer zone, flesh-pink; inner core, grayish black. Hardness, 
both can be cut with knife, easily. 

Draw No. 8, taken at 47y 2 hours. Temperature 1080°C. Outer 
zone three-quarters inch thick, inner core about 1 inch x 2V 2 , 
oval. Color, fair red exterior, light bluish black core. Hard¬ 
ness, materially greater, core harder than exterior, but both 
can be cut. 

Draw No. 9, taken at 48 hours. Temperature 1100° C. A 
marked physical change has taken place, more than commen¬ 
surate with the change in time or temperature. Probably 
explained by uneven distribution of heat in the first or larger 
kiln. Shrinkage has set in strongly. Vitrification marked. 
Cannot cut either red outer zone or blue core. Area of latter 
considerably smaller, and margins definite. Body at its densest 
state. 

Draw No. 10, taken at 49 hours. Temperature 1090°C. The 
shrinkage shown in draw 9 has been more than offset by the 
beginning of swelling. The central black core is much larger 
than No. 9, but this is not due to its expansion, at least not to 
any large degree. The core was larger when the advancing 
vitrification overtook it and prevented further oxidation. The 
core has swollen somewhat, however, and is microscopically 
vesicular. The outer red zone is still dense and not overfired, 
though very hard. 

Draw No. 11, taken at 50 hours. Temperature 1105°C. The 
swelling of the core is becoming clearly evident. It has de¬ 
formed the outer red zone, which is as yet not affected by over¬ 
fire, and it has become so coarsely vesicular that the bubbles 
can be seen by the eye. 


32 


THE N. B M. A. COMMITTEE 


Draw No. 12, taken at 52 y 2 hours. Temperature 1180°C. 

The process of swelling has progressed, and has now involved 
the exterior red zone as well. The black core is coarsely 
vesicular. The red exterior is finely vesicular. 

Draw No. 13, taken at 53 ^ hours. Temperature 1210 o, C. A 
portion of the trial only is shown. The trial had swollen enor¬ 
mously and could no longer be broken squarely across, but 
broke into irregular sharp jagged masses, like slag or glass. 

The piece shown exposes the red vesicular sponge of the outer 
layer, and a small part of the still more vesicular spherical 
core. 

Draw No. 14, taken after the muffle had cooled down to 
800° preparatory to receiving its brickettes for Burn B. The 
temperature did not go above that of Draw 13, but was of some¬ 
what longer duration. The sample represents the spherical 
black core only. It is a light scoriaceous cinder, which will 
float in water for hours. The exterior red vesicular mass has 
cracked away from the core, like the shell from a kernel. The 
red portion is not as coarsely bubbled as the black, but it is 
equally weak and worthless. 

The only variation from the program was that between the 
forty-seventh and forty-ninth hours, after the bricks in the kiln 
had partially puffed up, the temperature was held uniform for 
two hours, to show the effect of heat-soaking. This burn was 
completed at the fifty-third hour, trial No. 14 being the last 
sample. The muffle kiln was then cooled down to the tempera¬ 
ture of the large kiln, and prepared for Burn B. 

As the samples in the large kiln were not thoroughly oxidized 
yet, they were still treated in the large kiln for ten hours, when 
the charge for Burn B was transferred at the sixty-first hour. 
This burn was then carried out strictly according to program. 
The time temperature curve with indications of the points at 
which trials were drawn is shown in Figure 2. Also, photographs 
of typical trial pieces of the burn are shown in Figure 4. This 
burn was completed at the eighty-second hour, No. 30 being the 
last trial-piece to be drawn. 

Draws 1, 4, 5, 7, described under Figure 3. 

Draw No. 17, taken at 57 hours. Temperature 790°C. The 
sample was still oxidizing in the downdraft kiln. No important 
change in physical condition since sample 5 was taken. A very 
small black core still existed, and the burn was carried on 5 
hours longer, before transferring charge to updraft kiln for 
special treatment. 

Draw No. 19, taken at G8 y 2 hours. Temperature 980°C. 


33 


OX TECHNICAL INVESTIGATION. 












































34 


THE N. B M. A. COMMITTEE 


Just beginning to show a little hardening, but still cuts with 
knife, readily. Color uniform red. Shows a slight shrinkage. 

Draw No. 21, taken at 74 hours. Temperature 1'060°C. 
Much harder, but can still be cut a little. Shrinkage nearly 
complete. Fracture still granular. Color, good red throughout. 

Draw No. 22, taken at 75 hours. Temperature 1070°C. 
Shrinkage complete. Fracture denser and less granular, but 
not fully vitrified. Color, good red throughout. 

Draw No. 23, taken at 75% hours. Temperature 1075°C. 

No change in volume or color. Fracture becoming smoother 
and less granular. 

Draw No. 25, taken at 78 hours. Temperature 1075°C. No 
change in volume or color. Fracture smooth and glossy in 
most part. Cracks or lamination flaws beginning to open up in 
the mass. Past the best vitrification point. 

Draw No. 26, taken at 79% hours. Temperature 1100°C. 
Fracture smooth and glossy. Color, brown, uniform through¬ 
out. Cracks opening wider. Obviously deteriorating from over¬ 
fire. No change in volume as yet. 

Draw No. 27, taken at 81 hours. Temperature 1125°C. The 
swelling greatest on the exterior half inch, where the fracture 
is fine grained vesicular. Interior still smooth and glossy ex¬ 
cept for flaws, and does not show vesicular structure yet. 
Color, dark brown-red. 

Draw No. 28, taken at 81% hours. Temperature 1125°C. 

'Still further swelling. Vesicular structure worst and coarsest 
on exterior, but plainly visible clear through the mass. Cracks 
and flaws widening as the mass swells. 

Draw No. 30, taken at 82% hours. Temperature 1180°C. 
Body degenerated into a shapeless froth or sponge, which floats 
on water. Volume three or four times original size. Color, 
dark brownish red. Does not show any ferrous color or reduc¬ 
tion. 

The muffle kiln was then cooled down to about 800°C, and, 
in the eighty-fourth hour Burn C was started. This also was 
carried out according to program. Trial No. 34, drawn at one 
hundred and three hours, showed a good dense structure. Re¬ 
duction treatment was then applied. The wicket of the muffle 
was opened and about one pound of bituminous coal in small 
lumps was thrown in. The wicket was then quickly closed. The 
muffle immediately became filled with dense black smoke. The 
temperature was kept uniform. More coal was thrown into the 
muffle an hour and a quarter later, and the muffle kept filled with 
dense black smoke. Trial No. 37, drawn at the one hundred and 
sixth hour, showed black or blue vitrification extending deep 


ON TECHNICAL INVESTIGATION. 


35 


into the brick, a small red spot being left in the center. The heat 
was then raised, the ware still being kept under reducing condi¬ 
tions. The burn was completed at the one hundred and tenth 
hour, trial No. 40 being allowed to cool in the kiln with the 
damper down and the fire gases passing out through the muffle 
to ensure final reduction. The time temperature curve of Burn C 
is shown in Figure 2. Photographs of the typical trial pieces 
are shown on Figure 5. 


36 


THE N, B. M. A. COMMITTEE 



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ON TECHNICAL INVESTIGATION. 


37 


* 


Draws Nos. 1, 4, 5 and 7, described under Figure 3. 

Draw No. 31, taken at 98 hours, at temperature 1080°C. 

The bnickette had attained a fairly complete vitrification. Was 
too hard to cut, and of clean red color throughout. Shrinkage 
about complete. Reduction not yet begun. 

Draw No. 32, taken at 98^4 hours. Temperature 1080°C. 

The reducing treatment had begun, and a blue band had formed 
about %-inch deep on exterior of (brick. Meanwhile, shrinkage 
had gone on still further, to completion. 

Draw No. 33, taken at 102 y 2 hours. Temperature 1075°C. 

The second reducing treatment had begun. Sample was blued 
deeply, but some normal red color still existed in the center. 

No vesicular structure as yet. 

Draw No. 34, taken at 103 hours. Temperature 1'090°C. 
Blueing process going on steadily. No swelling yet begun. 
Body dense. Glossy fracture. 

Draw No. 35, taken at 104 hours. Temperature K)80°C. 
Brick beginning to show large cracks due to internal flaws and 
shrinkage defects. No swelling yet apparent. Body smooth 
and glossy fracture. Blue color on exterior. 

Draw No. 36, taken at 105 hours. Temperature 10i60°C. 
Vitrification very far advanced. Body breaks in smooth glossy 
fractures, very few grains apparent. Color blue on exterior. 
Swollen somewhat, chiefly iby opening of flaws and cracks in¬ 
side. Not vesicular, to the eye. 

Draw No. 38, taken at 10714 hours. Temperature 1040°C. 
Swollen badly, beginning at outside and progressing imwards. 
Soundest body in center. Color blue, with a small red core. 

Draw No. 39, taken at 108 x /4 hours. Temperature 1100°C. 
Exterior, coarsely vesicular, and burnt to a cinder *4-inch 
thick. Interior, vesicular, but in fine even texture. Cracks 
and flaws opened widely. Color blue -throughout. 

Draw No. 40, taken at 110 hours. Temperature 1130°C. 
Whole body very vesicular. Exterior notably worse than in¬ 
terior. Swelling has trebled volume of brickette. Color blue 
throughout. 

A study of these three series of trial pieces shows conclu¬ 
sively that the practical understanding and control of the burnin 
process for the clay had been worked out in the preceding year 
work as detailed in the preceding report (second), for three 
types of structure were produced at will, viz.: 

(a) The black-cored, swelled brickette, with red exterior. 

(b) The red brickette, with good dense structure until over¬ 
fired, and with vesicular structure proceeding from outside, 
inwards, when overfire set in, and with preservation of the red 
color throughout. 


yT crc. 


88 


THE N. B. M. A. COMMITTEE 


(c) The red brickette, with good dense structure, changed 
to a black cindery mass from outside, inwards, until on over 
heating, the whole mass was a vesicular sponge with black exterior 
and partially red interior. 

The very fact that these three characteristic varieties of 
color and physical structure could be produced with certainty 
and on the first attempt, shows clearly that the practical control 
of the burning process can be secured by following the methods 
prescribed as the result of the preceding year’s work, and to 
that extent it adds weight and authority to the opinions already 
announced in that place. 

IV, THE CHEMICAL WORK. 

Sampling. The number of samples secured in these various 
burns were so large that it was impossible in the time available 
to make analyses of all, nor did it seem that the comparatively 
small differences in degree between the changes in adjacent 
samples warranted the expense of separate chemical tests. There¬ 
fore the following samples were selected as types of the various 
stages of the processes represented, viz., the raw clay and draw 
trials 1, 4, 7j, 9, 14, 21, 22, 25, 28, 29 and 37. 

The raw clay and draw trial No. 14 consisted of one kind 
of material only, and were therefore prepared by pulverizing 
through a 100 mesh screen, mixing thoroughly, and putting away 
in stoppered glass bottles. 

All of the other samples represented two sets of conditions 
or two kinds of structure, and were therefore carefully broken, 
and the requisite quantity of both the inner and outer portions 
were taken separately, and two samples prepared. The distin¬ 
guishing of the inner and outer portions usually was very easy 
by observing the colors. In other cases, the brickettes were 
broken, piece by piece, and the material divided into outer and 
inner layers as fast as broken. All samples were carefully pul¬ 
verized and put up in stoppered bottles. 

The nature and peculiarity of each test piece can be easily 
found by consulting the description of the draw trials, under 
figures 3, 4 and 5, and can be still better appreciated from a 
study of the photographs of the more characteristic ones, which 
are printed herewith. 


ON TECHNICAL INVESTIGATION. 


39 



Figure 6. Showing cross section of Draw trial No. 1, drawn at 
710°C after 17 hours in the kiln. 

• 

The carbon has been burned out to a depth of about 5-16 inch, 
leaving the surface flesh pink in color. The core is very dark. The 
texture of both portions is very soft and crumbly. 

ANALYSIS. 



Cat bon 

Ferrous 

Oxide 

Ferric 

Oxide 

Total 

Sulphur 

Outer Oxidized Portion . 


0.31 

5 02 

0 94 

Interior Unoxidized Portion 

2.31 

4.66 

0 67 

2.40 























40 


THE N. B. M. A. COMMITTEE 



Figure 7. Showing cross-section of Draw trial No. 4, drawn at 
805°C, after 31 hours. 

0 

The carbon has been burnt out to a depth of y 2 or % inch. The 
outer portion is pink in color. The black core is much lighter and 
grayer than Fig. 6. Texture of both portions is still soft. 

ANALYSIS. 



Carbon 

Ferrous 

Oxide 

Ferric 

Oxide 

Total 

Sulphur 

Outer Oxidized Portion. 


0.36 

5.23 

0 68 

Interior Unoxidized Portion. 

1.173 

5.49 

0.62 

2.28 

























ON TECHNICAL INVESTIGATION. 


41 



Figure 8. Showing cross--section of Draw trial No. 7, drawn at 
980°C, after 44 hours. 

The carbon is burnt out to a depth of % or % inch, leaving the 
surface a flesh-pink color. The core is grayish iblaek. The texture is 
much harder, but, can still be cut with a knife. The shadows in the 
picture olbscure a considerable part of the core, which can be discerned 
only in its upper half. 


ANALYSIS. 




Ferrous 

Ferric 

Total 


Carbon 

Oxide 

Oxide 

Sulphur 

Outer Oxidized Portion. 


0.56 

5.33 

0.53 

Interior Unoxidized Portion. 

0.151 

5.34 

0.61 

2.15 

























42 


THE N. B. M. A. COMMITTEE 



Figure 9. 'Showing cross-section of Draw trial No. 8, drawn at 
108'0°C, after 47 y 2 hours. 

The carbon was tested in the black core but none was found. The 
exterior of the brick is a fair red color. The center is light bluish 
black. A marginal ring of a bright, different shade of red from the 
exterior encircles the black core. The exterior is fairly hard but has 
not lost its granular appearance. The red ring and the black area are 
more vitrified and break into a smoother fracture than the exterior, as 
can be seen. Not analyzed for other ingredients than carbon. 






ON TECHNICAL INVESTIGATION. 


43 



Figure 10. Showing cross-section of Draw trial No. 9, drawn at 
1100°C, after 48 hours. 

The size of the core and its relatively advanced degree of vitrifica¬ 
tion was probably not due to sudden changes occurring in the short 
period since Draw No. 8. It was probably due to irregularities of dis¬ 
tribution of the air currents and possibly of temperature in the first 
kiln, by which some of the brickettes were further advanced when they 
were transferred to the second kiln for special treatment. 

This brickette has passed beyond the most favorable stage of 
vitrification; the exterior in only a little spongy, but the center has 
swelled and deformed the exterior. The center is visibly spongy. 
The brick is hard and cannot be cut. 

ANALYSIS. 



Ferrous 

Ferric 

Total 

Carbon 

Oxide 

Oxide 

Sulphur 

0 

0.95 

5.90 

0.32 

0 

5 31 

0 36 

2.06 


Outer Oxidized Portion 
Interior Unoxidized Portion 


























44 


THE N. B. M. A. COMMITTEE 



Figure 11. Showing spherical core of Draw trial No. 14, after the 
red exterior shell had been cracked loose and removed. 

Taken after the kiln had cooled down to 80'0° preparatory to 
Burn B. The trial had not endured any temperature higher than No. 13, 
which was drawn at 1210°C, but it had stood a slightly longer exposure 
to heat. 

Both exterior and interior are light scoriaceous cinders. 

ANALYSIS. 



Carbon 

Ferrous 

Oxide 

Ferric 

Oxide 

Total 

Sulphur 

Inner Un oxidized Portion. 

Outer Oxidized Portion, not tested 

0 

6.15 

0 

1.34 



• 































ON TECHNICAL INVESTIGATION. 


45 



Figure 12. Showing cross-section of Draw trial No. 21, drawn at 
1060 e C, after 74 hours. 

The trial was fully oxidized, there being but one uniform red color 
throughout. Sample was getting hard but. could still be cut with a 
knife. 


ANALYSIS. 


Ferrous Oxide . 0.57 

Ferric Oxide . 5.43 

Total Sulphur .. 0.29 















46 


THE N. B. M. A. COMMITTEE 



Figure 13. Showing cross-section of Draw trial No. 25, drawn at 
1076°C, after 78 hours. 

1 he ti ial was smooth and glossy in fracture for the most part, 
the color was a good red throughout. Cracks or laminations are 
beginning to open in the mass, indicating the beginning of over-fire. 
No swelling as yet. 


Ferrous Oxide 
Ferric Oxide . 
Total Sulphur 


ANALYSIS. 


0.61 

5.54 

0.23 








ON TECHNICAL INVESTIGATION. 


47 



Figure 14. Showing cross-section of Draw trial No. 28, drawn at 
1125°C, after 81 y 2 hours. 

Notable swelling Ims taken place. The vesicular structure is 
worse on the exterior and least in the center, but can be observed all 
through the mass. Flaws and laminations spreading open widely. 

ANALYSIS. 


Ferrous Oxide . 0.61 

Ferric Oxide . 5.54 

Total Sulphur . 0.22 














48 


THE N. B. M. A. COMMITTEE 



Figure No, 15. Showing piece of Draw Trial No. 30, Burn B. This 
was a frothy cinder, light enough to float on water, but it was red in 
color, showing that even this high degree of fusion had not affected 
the status of the ferric iron. This sample shows the extreme point of 
breakdown of a ferrugineous clay, which had been properly oxidized 
and properly vitrified, and then subjected to normal over-fire. The 
expulsion of sulphuric acid is thought to be the cause of the highly 
spongy structure. This sample was not analyzed, hut No. 29, showing 
almost identical physical structure, contained 0.17 total sulphur, while 
No. 28, taken just before the heavy swelling began, showed 0.22 sulphur. 











ON TECHNICAL INVESTIGATION. 


49 





Figure 16. /Showing cross-section of Draw trial No. 37, drawn at 
1030°C, after 106 hours. 

Vitrification complete. Body breaks in smooth glossy fractures. 
Color is blue or black externally, with small red interior spot, not yet 
reduced. The rough granular area seen in the cut is a “dry spot,” or 
a break along a pre-existing flaw in the brick, and does not mean that 
the real texture of the brick is granular at all. From this trial, swell¬ 
ing developed rapidly as the temperature went up. 


ANALYSIS. 




Ferrous 

Ferric 

Total 

0 

Carbon 

Oxide 

Oxide 

Sulphur 

Exterior Reduced Portion. 

0 

6.26 

0.35 

0.26 

Interior Red Portion. 

0 

0.11 

6.55 

0.21 




























50 


THE N. B. M. A. COMMITTEE 



Figure 17. Showing a cross-section of Draw trial No. 39, drawn at 
1100°C, after lOS 1 /^ hours. 

The exterior has slagged badly to a depth of a quarter inch. The 
interior is vesicular and swollen, but not coarsely, and the fractures 
appear smooth and vitrified unless examined closely. Color is blue 
throughout. Not analyzed. 












ON TECHNICAL INVESTIGATION. 


51 



Figure 18. Showing cross-section of Draw trial No. 40, drawn at 
1130°C, after 110 hours. 

Color, blue or black throughout. Whole body very vesicular, 
swollen to a sponge, inside and outside. The swelling has about trebled 
the volume of the brick. This was the last brickette in Burn C. 















52 


THE N. B M. A. COMMITTEE 


CARBON, 


Method of Analysis. The powdered samples were first 
ground still finer to an impalpable powder in an agate mortar. 
One gram was taken for analysis. This was digested at a gentle 
heat in a mixture of 15 c. c. hydrofluoric and 5 c. c. sulphuric 
acids till practically everything except carbon was dissolved. 
The liquid and residue were then washed into a beaker containing 
200 c. c. of cold boiled water in order to dilute the hydrofluoric 
acid, thus rendering it safe to work with and at the same time 
preventing it from attacking the asbestos filter used in the next 
step. The liquid was then filtered through purified asbestos, in 
a Gooch crucible. The filter and carbon were washed with cold 
boiled water and dried. 

The asbestos and carbon together were then ignited in a 
combustion train. The train used was one of the ordinary kind 
with a twenty-burner furnace and a Jena glass tube, in which 
the sample is ignited in a slow stream of pure oxygen. The 
products of combustion are first passed over red hot copper 
oxide in order to convert any carbon monoxide (CO) into carbon 
dioxide (00 2 ) ; secondly, over hot lead chromate which holds 
any sulphuric or sulphurous acid present forming lead sulphate, 
and which also retains a part of any hydrochloric acid which 
might be present, forming lead chloride; thirdly, over silver foil, 
which catches and hold any chlorine escaping the lead chromate, 
forming silver chloride; fourth and finaUv, over calcium chloride 
which takes out any moisture present. 

The gases were then passed through a weighed set of Liebigs 
absorption tubes containing potasium hydroxide solution, sp. gr. 
1.27, and a guard tube containing soda-lime. These absorb the 
carbon dioxide in the gases. The increase in weight of the bulbs 
and guard tube is considered to be carbon dioxide, and from this 
the weight of carbon is calculated. 

It was found that the carbon in the samples burned unus¬ 
ually slowly, and without appreciable glow, so they were kept at 
a bright red heat for forty-five minutes to insure complete com¬ 
bustion. The ordinary time allowed for a combustion of the 
graphitic carbon in pig irons or steels is only fifteen minutes. 

Results. No question exists in the mind of anyone as to the 
absence of carbon from the red, so-called oxidized portions of 


ON TECHNICAL INVESTIGATION. 


53 


bricks. It is only suspected of being present in the black dis¬ 
colored portions, or cores, where it is thought by some to be the 
direct cause of the dark coloration. Accordingly, only a few of 
the samples were examined for carbon, and in those, only the 
dark interior core , not the red exterior. In one case only, Draw 
No. 37, was the exterior tested for carbon, and this was where a 
previously red brick was heavily and continuously reduced and 
blackened on the surface for a half inch or more, and in which 
the presence of some deposited carbon would have been not un¬ 
natural. None was found, however. 


Fig. 19. Showing Results of Carbon Analysis. 


Appearance of 
Sample 










Description of Trial Piece from which 
Analysis was made 

Carbon 

Determinations 

Raw Clay. 

2.870 

2.872 

Draw No. 1. Thin Exterior Shell 
of Pale Red. Core large and very 
black Drawn at 710 degrees. 173^ 
hours from beginning burn. Very 
soft. 

2.310 

2.310 

Draw No. 4. Exterior shell thicker 
and little stronger red. Core large, 
but paler color than 1 Drawn at 
805 degrees C., 3134 hours from be¬ 
ginning of burn. Very soft. 

1.100 

1.170 

1.190 

Draw No. 7 Exterior shell much 
thicker. Fair red. Core small and 
very pale color. Drawn at 98" de¬ 
grees C. in 44 hours from beginning 
of burn Not vitrified. 

0.150 

0.152 

Draw No. 8. Exterior shell vitri¬ 
fied red. Core, vitrified, dark blue 
color, and more glassy than red por¬ 
tion. Drawn at 108(3 degrees C in 
47 34 hours from beginning of burn. 

0 

Draw No 9. Exterior a little 
spongy from over-fire, red. Core 
considerably swollen and black. 
Drawn at 1100 degrees C in 4834 
hours from beginning of burn. 

0 

Draw No. 14. Spongy black core 
from which the red shell has been 
cracked loose. Enormously swollen. 
Drawn at 1210 degrees C. in 53 
hours from beginning of burn. 

0 

Draw No. 37 Exterior black from 
heavy reduction. Interior still red. 
Both are dense and break in glossy 
fractures. Exterior was analyzed. 
Drawn at 1030 C. in 10G>^ hrs. 

0 


Average per 
cent. Carbon 


2.874 


2.310 


1.173 


0-151 


0 


0 


0 


0 










































































ON TECHNICAL INVESTIGATION. 


55 


Rate of Loss of Carbon. The raw clay contained a very 
high amount of carbon to attempt to work. Shales of the same 
formation often contain 5 or even 10% of carbon, but are then 
considered practically unworkable. 

Sample No. 1, drawn at 710°C, showed a black friable core, 
containing 80% of the initial carbon. Sample No. 4, drawn at 
803°C, showed a little harder core, a little less glossy black, and 
having a carbon content of only 40.7 % of the initial amount in 
the raw sample. Sample No. 7, drawn at 980°C, shows a grayish 
black core, one inch in diameter, and having only 5.2% of the 
initial amount, or in actual percents, 0.15 of 1% of the weight of 
the brick. Just where the last of the carbon goes out is not 
known, but sample No. 8, drawn at 1080° C, showed a black 
vitreous interior in which there was no carbon found, and 
there was no sign of puffing or vesicular structure, in either red 
exterior or black core. The appearance of this brick is well shown 
in Figure 9. 


IOO 


90 


8o 




70 


60 


<0 
& 

1 

8 
g 

^ * 

* 

<0 
<0 

50 

-s 


30 


20 


10 


V 


V -Jw '-'vy o u ou 

Duration of Heat Treatment, in Hours 
Fig. 20. 


rt—- 

6 > 

\ 

k S' 











$ 

\ — ^ 

\il 

V 




Cun/e stowing percentage of 
the initio/ carton remaining 
m tfie black cores of brickettes 
after various deviations of the 
beat treatment 





G 

1 






$ 

<0 

Jt 































G 

I G 

<0 <o 





G 

o 






\ CVJ 

N \ 

0 ) 





\ 

K 











i. 





\i 

^ 1 

> ^ 

J 





1 











































56 


THE N. B. M. A. COMMITTEE 



Relation between size and density of black cores. The rate 
of loss of carbon in this series of tests has brought attention to a 
point not properly appreciated before, at least by the writers, 
viz., that the size of the core is not a good gauge of the amount 
of the carbon it may be harboring. In this series of brickettes, 
the size of core changed but little from Draw No. 1 to Draw No. 5, 
but the amount of carbon was decreasing in these cores steadily, 
as was shown by the drop between Draw 1 and 4, 2.31% to 
1.17%. Similarly, between No. 4 and No. 7, although the core 
shrank one half in size, the carbon fell away by about nine-tenths, 
or 1.17 to 0.151. 

In this experiment, it was the design in Burn A to start the 
black coring reaction while plenty of carbon yet remained in the 
trials, and judging from the size of the cores we should have" 
succeeded. This method misled us, however, so that when we 
reached vitrification, our carbon had wholly disappeared. It had. 
however, left a black core in its place, but one due to other 
causes, to be discussed later. 

Negative evidence on Coring by carbon. The results 
reached, therefore, do not settle the point as to whether the black 
cores in vitrified bricks may not be due to actual carbon enclosed 


Wo /■+ - S3 2 hours 








































ON TECHNICAL INVESTIGATION. 


57 


in them. They show that no carbon existed in ihese bricks when 
they reached vitrification;, although black cores of large extent 
remained, and later developed their characteristic effects. They 
confirm the analyses made by Prof. Lord for this committee, 
quoted on page 14, in that when the wares reached vitrification, 
no carbon remained. But the fact that the analyses show that 
the cores were already reduced to low carbon content (about 1%) 
before the heat was raised above 800° C, and the fact that the 
brickette contained only 0.151 carbon just before reaching vitri¬ 
fying temperatures, does not enable us to say with authority that 
carbon may not be found as to the colorant in some vitrified 
clay wares. 

Hopwood* and Jackson found carbon in the black non- 
vitrified portions of abnormally fired clay wares. However, the 
samples examined by them were not, as far as we know, bloated, 
or near the bloating stage. We are willing to admit the proba¬ 
bility of the truth of their statement that “the black internal 
colouration of abnormally fired clay wares is due principally to 
free carbon” when the statement is made in reference to very 
soft-burned wares, or to wares that have been scarcely oxidized 
at all, but do not think that it applies to hard-burned wares that 
have been tolerably well oxidized, such as No. 2, or the puffed 
samples up to No. 14. 

Theory as to the disappearance of the carbon from seemingly 
vitrifijsd samples, without swelling. In the report of last yea.iyj- 
the theory was tentatively put forth “that the carbon remaining 
in the clay at the beginning of vitrification was converted into 
carbon monoxide or dioxide by reduction of the ferric or ferrous 
oxide, and held in the clay under pressure, and which therefore 
would escape in grinding the sample preparatory to making an 
analysis, and thus would not show up in an analysis as free 
carbon. When the temperature was raised to a point at which 
the clay began to soften, the constantly increasing pressure of the 
gases would cause the softened mass to swell and develop a vesi¬ 
cular structure.” 

This theory has received further consideration, and the fol¬ 
lowing analysis of the conditions of the case has been made: 

Suppose all the carbon did not go out during oxidation and 

♦Trans. North Staffordshire Ceramic Society, 1902-1903, page 105. 

flnfluence of carbon in the burning of clayware, 2nd Report, Com. 
on Tech. Inves., N. B. M. A., 1904. 



58 


THE N. B. M. A. COMMITTEE 


that 0.1 of 1% of the weight of the brick consisted of free carbon, 
and that this was converted into carbon monoxide. The brick 
weighs approximately 1500 grams. One-tenth of one per cent, of 
1500 grams ~1.5 grams of carbon. AVliere carbon monoxide 
(CO) is formed, 12 parts of carbon unite with 16 parts of oxygen 
to form 28 parts of carbon monoxide. Then by proportion, 

12 : 28=1.5 : X 

X=3.5 grams of carbon monoxide formed. 

One liter of carbon monoxide at 0°C=1.234 gr. (Smith¬ 
sonian Physical Tables). 

By calculation, one liter of carbon monoxide at 1000°C and 
ordinary atmospheric pressure=0.2643 gr. 3.5=.2643=13.20 
liters of carbon monoxide at 1000°C, and one atmosphere (14.7 
pounds) pressure. 1 liter equals 61 cu. inches. 13.20X61=805.2 
cubic inches. 

Since the brickettes used in this investigation measured 
about 3x4x4 inches or 40 cubic inches, the gas evolved by the 
oxidation of 0.1% of carbon would have made a bulk, under 
normal atmospheric pressure, of 805.2=40, or 20 times the vol¬ 
ume of the brickette. If 10% of the sample were free open pore 
space, the volume of gas would be 200 times as large as the space 
available. 

As the volume of a gas varies directly with the temperature 
and inversely with the pressure, and if the volume were main¬ 
tained constant, and the temperature were increased, the pressure 
would vary directly as the temperature, then it follows that in 
the case above cited the pressure in the cold brickette would be 
according to the proportion: As the volume of the pore spaces 
are to the free volume of the gas generated, so is free pressure of 
the atmosphere to the pressure generated; or, expressed in figures, 

1:200:: 14.7: X 

where X is equal to the pressure that would be necessary to com¬ 
press the gas from 200 volumes down to one volume. On solving 
the proportion, this is found to be equal to 2940 lbs, per square 
inch. 

If this theory were sound, the gas generated by the reduction 
of the iron would be held under this enormous pressure in a 
brickette which had been chilled down to the atmospheric tem¬ 
perature from the partially vitrified condition, without giving 
any sign of its presence, which does not appear reasonable. It 
seems, therefore, that this theory is not tenable, and that if either 



ON TECHNICAL INVESTIGATION. 


59 


carbonic oxide or acid is generated by reduction of iron or other¬ 
wise, that it must speedily escape through the pore system or else 
swell the clay and produce a vesicular structure. The absence of 
carbon by analysis must be construed as proof of its escape from 
the clay. 

Deposition of carbon by heavy reduction. A test was made 
on the black exterior of No. 37 for carbon but none was found. 
Carbon may be deposited from fire gases into the pores of a 
porous clay ware, as familiarly seen in the German “Blue 
Smoked” roofing tile, but it is hard to see how it could get into 
the interior of a vitrified ware to any large extent. 

Summary. The results of the work done on the carbon may 
be summarized as follows: 

I. The carbon was responsible for the color of the dark 
cores of this clay during the early portion of the burn while the 
ware was still very soft and porous. 

II. The carbon in this core burned away steadily during the 
oxidation and early vitrification period, leaving none in the clay 
after complete vtrification had occurred. 

III. Ten other samples of black cored or reduced clay 
wares of various sorts showed no carbon present in the dark 
colored spots. 

IY. In this clay, when the carbon had been recently ex¬ 
pelled at the time when complete vitrification was reached, the 
brickettes formed black cores of shape and extent similar to that 
recently vacated by the carbon. 

V. The active or immediate cause of these black cores must 
be sought outside of the carbon, which seems to be generally 
absent when the core is formed. 

VI. The carbon in a vitrified body could not be converted 
into gas by oxidation without causing vesicular structure to 
develop. But dark colored vitrified cores do develop constantly, 
without showing any signs of vesicular structures for long per¬ 
iods after formation. This seems to still further militate against 
the presence of carbon being a direct cause of the black color¬ 
ation. 

VII. That in the case of dark coloration produced by heavy 
reductions in carbonaceous atmosphere in a body previously well 
oxidized, no carbon was found in the dark areas, and the cause 
of the color must be sought elsewhere. 

VIII. That authentic cases of the deposition of carbon in 


60 


THE N. B M. A COMMITTEE 


the interior of clay wares, by long continued heating in heavily 
reducing atmospheres, can doubtless be shown, but that in such 
cases, the clay wares are porous, and the reduction is heavier than 
is likely to be produced in ordinary kiln firing, and the deposition 
of carbon is obtained by special treatment, designed for the 
purpose. 

IRON. 

The status of this element being in serious question, a con¬ 
siderable number of determinations were run, on total iron, and 
ferrous oixde, the ferric oxide being obtained by difference. 

Method of Analysis. Total Iron. A one gram sample was 
fused with ten times its weight of dry sodium carbonate. The 
fusion was digested until completely disintegrated in warm 
water, was then acidulated with hydrochloric acid and evaporated 
to dryness. The residue was moistened with hydrochloric acid 
and the soluble portion was dissolved in warm water, and the 
silica was filtered off and thoroughly washed. Sulphuretted 
hydrogen gas was then passed into the filtrate to precipitate any 
platinum that might have been dissolved from the crucible during 
the fusion. The solution was filtered, boiled to drive off the 
excess of sulphuretted hydrogen, filtered again to remove free 
sulphur, and then made alkaline with ammonium hydroxide, and 
boiled. The precipitate, consisting of hydroxides of iron and 
aluminum, was allowed to settle, the supernatant liquid was 
decanted off, and the precipitate was washed onto a filter. The 
precipitate was dissolved again in hot dilute hydrochloric acid 
and collected in a clean beaker. The solution was diluted to 
100 c. c., and heated to boiling. The iron was reduced by 
stannous chloride, added drop by drop. The excess of stannous 
chloride was then neutralized by adding an excess of mercuric 
chloride. Then the iron was determined by titrating with a 
standard solution of potassium bi-chromate. 

This method is considerably longer than the ordinary iron 
determination. All of the steps between the separation of the 
silica and the reduction by stannous chloride are generally 
omitted. In fact, some chemists do not even separate the silica. 
We tried to use the solution obtained immediately after the 
separation of the silica, but found that our results were high. In 
order to get our hard burnt vitrilied clays into solution it was 
necessary to use ten grams of sodium carbonate, and to heat it to 


ON TECHNICAL INVESTIGATION. 


61 


the highest limit of the blast lamp for ten or fifteen minutes. 
The strong alkaline fusion always attacked the crucible, appre¬ 
ciable amounts of platinum being separated by the proper treat¬ 
ment. 

Ferrous Oxide. A one grain sample was digested at a gentle 
heat in a mixture of 15 c. c. hydrofluoric acid, and 5 c. c. sul¬ 
phuric acid in an atmosphere of carbon dioxide. This was ef¬ 
fected in the following manner: The sample was placed in a 
platinum crucible and moistened with a little freshly boiled 
water, which had been cooled down to the atmospheric tempera¬ 
ture again. The crucible was placed in a support on the center 
of the bottom of an enameled iron pan, and the pan was placed on 
a tripod. A bell jar, improvised from a two liter bottle with its 
bottom cut out, was set down in the pan, over the crucible. The 
space between the bottle and the sides of the pan was filled in 
with fine white sand. The sand made a layer V/j inches wide and 
2 inches deep, and formed a fairly tight seal. The bottle was 
fitted with a two-hole rubber stopper. In one hole was placed 
a small separatory funnel with its stem leading down into the 
top of the platinum crucible In the other hole was placed a small 
piece of glass tubing to act, as an exit tube. The carbon dioxide 
was led in from a Kipp generator through the sand layer and 
under the edge of the bell jar by means of a piece of bent glass 
tubing, and the gas was discharged below the level of the top of 
the crucible. Being heavier than air, it filled the bottom of the 
space first, then gradually rose in the bottle until finally it over¬ 
flowed at the top through the exit tube. At the beginning of a 
determination, the gas was passed into the jar at a rather rapid 
rate till all the air was displaced and the gas coming from the 
exit tube would promptly extinguish the glow of a match. Then 
the mixture of hydrofluoric acid and sulphuric acid was allowed 
to run down through the funnel into the crucible. The flow of 
carbon dioxide was allowed to slacken to a small but steady 
stream, and a small flame was placed under the pan. The flame 
was so regulated that fumes of hydrofluoric acid were given off, 
but the sulphuric acid was not brought to its boiling point. When 
practically all of the sample had gone into solution, the glass 
cover was raised above the level of the crucible, without removing 
the flame or stopping the flow of carbon dioxide, the crucible was 
siezed with a pair of platinum tipped crucible tongs and quickly 


02 


THE N. B. M. A. COMMITTEE 


dropped into a beaker of cold boiled water. This solution was 
then immediately titrated with potassium bichromate solution. 
As there had been no chance for either oxidation or reduction of 
the iron, during solution, then the iron which reacted with the 
indicator represented ferrous oxide in the sample. 

The solution of the harder burned samples in hydrofluoric 
and sulphuric acids was a difficult thing to accomplish without 
danger of oxidizing some of the ferrous oxide to ferric oxide by 
means of the hot sulphuric acid. An ordinary rock analysis is 
considered rather difficult, when it is possible to disintegrate the 
sample with a mixture of acids heated to the boiling point of 
water. In our work, on account of the resisting power of hard 
burned vitrified clay powder, it was impossible to disintegrate 
tiie samples unless the acids were heated till the hydrofluoric acid 
had largely distilled off at 110°—120°C, but of course it was 
necessary to keep the temperature well below the boiling point of 
the sulphuric acid. 


Ferric Oxide. 'The total iron and the ferrous oxide being 
known, it was easy to obtain the amount of iron existing as ferric 
oxide by difference; it would be rather difficult to make a direct 
determination of it; and none was attempted. 

The Results. The figures obtained by the above methods 
from the samples before described are given in the following 
table: 


ON TECHNICAL INVESTIGATION. 


63 


Table No. 4. Results of Iron Analyses. 


Designation of Burn. 

Number of Draw-Trial 

Red Portion of Brickettes 

Black Portion of Brickettes. 

Amount of Iron 
existi g as Ferric 
Oxide expressed in 
terms of the metal. 

Amount of Iron 
existing as Ferrous 
Oxide expressed in 
terms of the metal. 

Total amount of 

Iron found ex¬ 

pressed in terms 
of the metal 

Amount of Iron 

existing as Ferric 

Oxide expressed in 

terms of the metal. 

Amount of Iron 

existing as Ferrous 

Oxide expressed in 

terms of the metal. 

Total amount of 

Iron found ex¬ 

pressed in terms 
of the metal. 

1 

Raw 

Olay 

1 

1.84 

1.79 

3.58 

3.58 

5.42 

5.87 

1 Calculated on basis of the burnt clay 
viz., allowing for a loss of 11.16% 
j of the clay it firing. 



5.02 

0.31 

5.33 

0.67 

4.56 

5.23 


1 

4 97 

0 31 

5.28 

0.70 

4.61 

5.31 



5.49 

0.36 

5.85 

0.62 

5 23 

5 90 


4 

545 

0.33 

5.79 

0.67 

5.18 

5.85 

< 








3 








£ 


5.34 

0.56 

5.89 

0.52 

5.33 

5.85 

pq 

7 

5.28 

0.56 

5.85 

0.52 

5.33 

5.85 



5.31 

0.95 

6 25 

0.36 

5.90 

6.25 


9 

5.23 

1.02 

6 25 

0.36 

5.90 

6.25 



No red 

color 

produced 


6.15 

6.15 


14 

in 

this 

sample 


6.15 

6 15 



5.43 

0.51 

5.95 





21 

5 45 

0.51 

5.97 






5 46 

0 61 

6.08 





22 

5.54 

0.61 

6.15 




M 








3 


5.54 

0.61 

6.15 

These bricks had 

no black 

f-i 

3 

25 

5.53 

0.64 

6.18 

r core 

or black exterior. 

ffl 










5.54 

0.61 

6.15 





28 

5.54 

0.61 

6 15 






5.54 

0.71 

6-25 





29 

5.51 

0 74 

6.25 

✓ 





6.55 

0.11 

6.67 

0.35 

6.26 

6.61 

3 U 

W 

37 

6.50 

0 11 

6.61 

0.30 

6.26 

6.56 


































































































































64 


THE N. B. M. A. COMMITTEE 


Analysis of the Results. The totals. The total amount of 
iron found in the different brickettes, and in the different parts 
of the same brickettes, is on the whole quite uniform, but there 
are some interesting discrepancies. In no ease in the 90 deter¬ 
minations is there any serious divergence between the total irons 
found in the red portion and the black portion of the same brick. 
The greatest is about 1.6% of the average amount of iron present. 
In most cases, the checks are very good. In some instances there 
were determinations made which departed widely from the gen¬ 
eral run, and these were remade until the trouble was cleared up 
and the determinations became consistent and duplicatable. 

When we consider the checks between different brickettes, 
the agreement is not so good. The clay from which the brickettes 
were prepared should produce a more homogeneous distribution 
of the iron than was found in this study. There seems no reason 
on the face of this case, why individual bricks should have varied 
more than 2 or 3% of the average contents of the batch. AVe 
hod as follows: 

TABLE No 6 


No. 


Average of Iron 
Determinations 

0 

BURN A. 

Raw clay . 

5 40 

1 

Softest, but little oxidized . 

5 29 

4 

Soft, partially oxidized . 

5.84 

7 

Much harder, small core . 

5.86 

9 

Vitrified, beginning to swell . 

6 25 

14 

Swollen to a sponge . 

6.15 

21 

BURN B. 

Oxidized, granular, soft .*.... 

5.96 

22 

Oxidized, granular, harder . 

6.12 

25 

Oxidized, vitrified . 

6.16 

28 

Oxidized, vesicular . 

6 15 

29 

.M 

Oxidized, very spongy . 

6.25 

37 

BURN C. 

Vitrified, not yet vesicular .. 

6.61 

























ON TECHNICAL INVESTIGATION. 


65 


There is a curious increase to be noted here between the 
raw clay and the burnt, and between the soft burnt and the hard 
burnt of each series, for which no adequate explanation can now 
be advanced. The natural explanation of the loss of volatile 
constituents has been forestalled by the calculation of the iron 
found in the raw sample into what it would be on the burnt 
sample, allowing 11.06% loss in heating. 

There are also discrepancies to be expected in the percentage 
of iron found in different trials because of its distribution in 
different forms in them, but these are small. The lower amount 
of iron in Sample 1 than the calculated figure of the raw clay is 
due to there still being the bulk of the carbon left in Sample 1, 
while the other figure is assumed on the basis that the carbon has 
been sent out. But the very marked increase in iron in all the 
succeeding samples cannot be explained on any such basis. 

Between samples 4 and 29, the fluctuations from the average 
are not beyond what might be termed ordinary working margins 
in analytical work. But No. 37 and No. 1 vary 1.32% or 21% 
of the above average figure. 

The natural explanation to give is that of extensive altera¬ 
tion by volatilization of the clay ingredients. This is suggested 
in samples 14 and 29, which are the two most vesicular samples 
of their respective burns that were tested, but this theory is un¬ 
tenable in view of the iron content found in No. 37, which was 

not vet vesicular. 

«/ 

'there is, of course, the hypothesis that the work was in¬ 
correct, and badly done. But as it was performed at leisure, by 
one man only, who spent all needed time on it and never left a 
sample until duplicate tests checked nicely, it is difficult to accept 
this idea as correct, (especially as in most of the cases, duplicates 
made from separate samples of different parts of the same brick 
showed on assembling, substantially the same amount of iron 
present). It, seems, therefore, an interesting question to carry 
through at a subsequent investigation. 

The Ferrous Determinations. The ferrous iron determina¬ 
tions were very nice and regular, so far as ability to check de¬ 
termination is concerned. These determinations did not give 
much evidence on the problem brought up by the totals, except 
in the case of No. 14 and 37, in which the ferrous iron alone 
closely checked the high totals of the same samples. 


66 


THE N B. M. A. COMMITTEE 


The Raw Clay. In the raw clay, 66% of the iron was in the 
ferrous condition. The mineral in which it occurs is probably 
ferrous carbonate, but there is also a considerable amount of 
pyrite and probably a little ferrous sulphate. 

The Distribution of the Iron in the Red Portion of the 
Brickettes. The iron seems to have changed over into the ferric 
form with comparative ease and completeness, as the carbon was 
burnt out and the clay took on its red color. In fact, the oxida- 
tion attained immediately after the removal of the carbon at 
temperature of 700°C and 800°0 was more complete than that 
observed as the temperature rose. Thus the percentages of fer¬ 
rous oxide remaining in the red portions of the various brickettes 
was: j 

TABLE No. 6. 


No. 


Per cents, of the Total 
Iron Present 

0 

BURN A. 

Raw . 

33 00 

1 

4 

At beginning of oxidation period. 

Oxidation period well advanced. 

5.81— 5.87 

6.15— 5.70 

7 

Vitrification period begun. Small black 



core . 

Q ^ Q Q 

9 

Vitrified and vesicular . 

v . 0 J V . O / 

15.20—16.30 

21 

BURN B. 

Core completely gone, bright red through 



out . 

8.57— 8.54 

22 

Harder but not vitrified . 

10.03— 9.91 

25 

Vitrified but not vesicular. Red. 

9.91—10.35 

28 

Vesicular, just beginning to swell strongly 

9.91 9.91 

29 

Vesicular, spongy mass . 

11.36—11.84 

37 

BURN C. 

Vitrified red center. Exterior black and 



reduced . 

1.64— 1.64 


The above facts are shown graphically in the following curve 
sheet, Figure 22. 


























ON TECHNICAL INVESTIGATION. 


67 



O 


)00 200 3oo 4-00 v5oo 6oo Too 3oo 3oo iooo lioo t2oo 

Temperature in Degrees Cenf/grac/e. 

Figure 22. 













































































68 


THE N. B. M. A. COMMITTEE 


A study of the above table and curves shows: 

1st. That the iron does not seem to go wholly into the ferric 
form, even after prolonged periods of firing in oxidizing atmos¬ 
pheres. It behaves in a manner analagous to carbonate of cal¬ 
cium, when heated, which loses 'C0 2 until but a small quantity 
remains which cannot be expelled without undue heat and con¬ 
ditions not commercially attainable. Many other illustrations of 
the same principle of chemical equilibrium are observed in chem¬ 
ical decompositions. Red clay wares, no matter how bright in 
color, are thus likely to contain some ferrous oxide. 

2nd. That increase of temperature above that favorable to 
oxidation (800°C) and running into the vitrification zone, seems 
to actually militate against the oxidation. A counter influence 
of some sort is set up, by which the proportion of ferrous oxide 
increases 50% over its minimum, or from 6% up to 9 or 10%. 
Here it seems to tend to remain, as samples of very varied physi¬ 
cal structure and degree of hardness clearly show. Numbers 
7, 21, 22, 25 and 28 illustrate this condition. 

3rd. That driving the temperature up to the point where 
the clay becomes markedly scoriaceous or vesicular seems to 
tend to still further increase the ferrous iron. This tendencv is 
not clearly proven, for in No. 29, which was very scoriaceous, 
though a good red color, the increase in ferrous oxide was small, 
while in No. 9, which contained a large black core, the red portion 
jumped up to 15 or 16% of ferrous oxide. It may have been 
influenced to do this by the passage of gases from the interior on 
their way out. Carbonic oxide (CO) or sulphurous acid (SO.,) 
might readily exert such an effect. 

4th. The fact that clays may fuse and become scoriaceous 
without thereby blackening or undergoing a breaking down of 
Fe 2 0 3 to 2 FeO as a necessary corrolary is clearly shown by these 
samples. This point is commonly illustrated in many other clays, 
but as the idea has been advanced by one of the present writers 
(Orton) that fusion means accompanying or preceding blacken¬ 
ing, it is perhaps worth while to call attention to the untenability 
of this hypothesis. 

The Black Portion of the Brickettes. The reducing agent 
which causes the black coring reaction, whatever it is, is more 
powerful in its operation than the oxidizing reaction which 
operated in the red colored areas, for the percentage of ferric 


ON TECHNICAL INVESTIGATION. 


69 


oxide in the black areas became constantly less with increasing* 
temperatures until none remained. It would seem that the pres¬ 
ence of either carbon or sulphuric matter intimately mixed with 
the clay mass is powerfully efficient as a dioxidizer of iron. It 
also appears that even when it is shown by analysis that no carbon 
remains, reduction of ferric iron to ferrous still takes place. 

Thus in draw trial 7, the amount of carbon found by test 
was 0.15% and the ferric iron in the black core was 0.52%. In 
No. 9, the carbon was 0, while the ferrous iron had fallen to 0.36. 
In sample 14, however, there was no carbon, but the ferric iron 
had now fallen to zero. The loss of 0.36% of ferric iron from 
the black core must therefore be accounted for without consider¬ 
ing carbon as a possible cause. The possible influence of sulphur 
as the cause of this reaction will be considered elsewhere. 


70 


THE N. B. M. A. COMMITTEE 



o 

o 

N 


O 

O 


O 

o 

o 


o 

o . 

<r> 

.1 


jo 

° ^ 

? 

1-5 

-*$ 

O 

Q S$ 
10 ^ 


I 


8* 

't 


o 




o 

o 

M 


O 

O 


Figure 2o 













































































ON TECHNICAL INVESTIGATION. 


71 


This reduction could not have proceeded from outside in¬ 
wards, because in every case there was a shell of red matter en¬ 
compassing- the black portion. In Burn C, one test brickette only 
was examined. We find it significant in two respects. 1st, its 
red portion is the nearest to completely oxidized of any sample 
in the whole test, there being one about 1.6% yet in the ferrous 
condition. 

It had been under oxidizing influences for nearly a hundred 
hours. In the reduced exterior, on the other hand, 95% of the 
iron was in the reduced condition. As all visible signs of reduc¬ 
ing agents had been burned out long since, and there was no 
deposition of carbon in the pores of the clay, this intense reduc¬ 
tion was no doubt due to the influence of the gases of the kiln 
atmosphere. As the pores of the clay were all or nearly all closed 
at the time, the action must have been an intermolecular one. 
the gases continuously taking the oxygen from the molecules on 
the surface, and these molecules taking the oxygen from the 
molecules next to them toward the interior, and so on till oxygen 
was finally being transferred from deep in the interior of the 
brick to the surface. 

The Hole Played by Iron. The role played by iron in the 
vitrification of clay wares is understood to but a limited extent 
as yet. We know in a general way that iron oxide reduces the 
temperature at which clays are burnt, i. e., that red burning 
clays, high in iron, are burnt at temperatures considerably below 
the buff burning clays low in iron, and still further below the 
white burning clays in which iron is practically absent. We 
know that it serves a useful purpose in this respect, and that the 
bodies produced by vitrification of ferrugineous mineral mixtures 
are often just as desirable in every way for the practical uses of 
life as the lighter colored ones, which require more fuel and are 
therefore more costly. But we have as yet little detailed and 
quantitative knowledge of the exact effect of iron in the various 
mineral mixtures which we call clays. 

There has been much speculative discussion about the effects 
of iron in clays. Much of it centers around the three following 
topics: 

I. The Black-Coring reaction. As an improperly oxidized 
clay “black-cores,” i. e., shows fusion and vesicular structure in 


72 


THE N. B. M. A. COMMITTEE 


the unoxidized area, is this premature fusion due to the state of 
oxidation of the iron? 

II. The Blue-Stoning reaction. As a properly oxidized 
clay gradually darkens in color when subjected to increasing 
temperatures during normal burning, is it due to changes in the 
state of oxidation of the iron? 

III. The final Slagging or Over-fire reaction. As a pro¬ 
perly oxidized clay slags and becomes spongy from over-fire 
under oxidizing conditions, is it due to changes in the state of 
the oxidation of the iron? 

These topics will be taken up in order, and studied from the 
point of view of the data now on hand. 

The Black-Coring Reaction. It has been stated by Wheeler 
that ferrous oxide becomes active in slagging the clay about 
200°C below the temperature at which ferric oxide would become 
an active flux. 

On the basis of this statement, an explanation of the black 
coring reaction has been built up by Orton and others, to the 
effect that this early or premature fusion in the unoxidized area 
of the clay, occurring while the clay is still disengaging gases 
from its interior parts, is the cause of the black swelling and 
scoriacious slag spots in the center. 

It was observed in this study, that the swelling of the brick- 
cttes from the black-coring reaction, and that which came from 
the simple overheating of the well-oxidized clay, both occurred in 
a very narrow temperature range—between 1080° and 1100°O. 
In both burns A and B, the swelling occurred while the kiln was 
being fired with a nice oxidizing atmosphere. In Burn C, which 
was fired under heavy reducing conditions, the first test piece to 
show clearly vesicular structure from over-heating was No. 38. 
which was drawn at 1060°C, but which had been fired rapidly up 
to 1100°C shortly before. When the remaining brickettes were 
again heated up from 1060° to 1100°, they promptly swelled 
and much more markedly than No. 38. 

It is safe to say, then, that in all three burns, puffing oc¬ 
curred between 1080° and 1100°C. In all three cases, the puffing 
occurred at the same stage in the vitrification process, a short 
time after the vitrification had reached the point of best develop¬ 
ment. In each burn, a sample with dense, solid glassy structure 
and almost no porosity was secured before samples showing signs 


ON TECHNICAL INVESTIGATION. 


73 


of vesicular structure began to appear. In samples having a red 
or oxidized portion and a black or reduced portion, the black 
portion became Ossicular a short time before the red portion. 
The black portions become more vesicular than the red also. In 
No. 9, the black core is noticeably vesicular while the red portion 
is minutely vesicular in some parts. In No. 37, the black part 
is minutely vesicular while the red portion is still sound, but the 
next sample drawn a short ime afterward is vesicular in both the 
black and red portions. 

We can say then that in this clay, the breaking down and 
development of vesicular structure occurred at a distinct but very 
slightly lower temperature in those portions in which the ferrous 
oxide was high and ferric oxide low than in those portions where 
the bulk of the iron present was in the ferric condition. 

Also, that in all cases, a dense, solid, aparently strong body 
of a blue color was obtained where the iron was predominately 
ferrous, exactly comparable with the dense solid strong body of 
a red color, where the iron was mainly ferric, and that these 
stages were produced and passed through before either portion 
became vesicular. 

On the other hand, instances are numerous and undeniable 
where red burning and buff burning clays do develop a vesicular 
core which swells and deforms the ware, while the encompassing 
normal portion of the ware is not in the least degree vesicular 
nor near its point of failure by over-fire. Observations of the 
product of commercial plants often discloses wares in which the 
exterior oxidized and normal portion of the ware in different 
parts of the kiln has been subjected to wide fluctuations of tem¬ 
perature, during which it has passed from a low grade of vitrifi¬ 
cation up to a high grade of complete vitrification—evidently 
covering considerable time and temperature interval—without 
showing any signs of over-firing, while the interior core has 
meanwhile undergone profound changes of density and degree of 
fluidity. This evidence is too well established and confirmed by 
too much industrial practice to be easily overthrown. 

We have, then, two dissimilar cases, 1st, the clay of the 
present study, which black-cores and swells at a low temperature, 
and at a very slight temperature interval before the red properly 
treated portion also swells and fuses. And 2nd, the general run 
of red- and buff-burning clays, such as are used for paving brick 


74 


THE N. B. M A. COMMITTEE 


and sewer pipe, which black-fore and swell a long, but not yet 
accurately measured interval, before the red portion begins to 
break down from over-fire. 

It should be said that there are undoubtedly many clays 
that do belong in the former class; many have been observed to 
give identical results in test-kiln firing, but we happen to possess 
no measured data concerning them. 

The question now arises—how can ferrous iron be respon¬ 
sible for early fusion in one case and not in another! Before 
answering this, another question should be asked, viz., are we 
really sure that ferrous iron is the cause of the early fusion ! In 
other words, is the fundamental statement of Wheeler as to the 
difference in melting point of the ferrous silicate true or not! 
There seems some reasons for doubting it. For instance, in 
America, many paving blocks are made with a blue or reduced 
exterior zone, sometimes half an inch or an inch in thickness, 
covering a red core of vitrified material. Such bricks are made 
by heavy reduction after the burn has been properly carried 
through the vitrification process. This condition was produced 
in our Burn C, in sample No. 37, and those following it. The 
chemical test of No. 37 shows that the iron was thoroughly re¬ 
duced by the gaseous treatment, and that practically no ferric 
iron remains in the blue zone, and there seems reason to expect 
a similar state of things in other clays showing the same color 
changes in response to the same treatment. Now these blue- 
coated pavers often give most excellent results for strength and 
toughness in the rattler and in street use. On examining them 
carefully, the blue portion is not necessarily found to be vesicular 
or defective in structure by reason of its color change, nor is 
there any evidence of greater fusibility than in the red or ferric 
portion. 

In England, blue-fired clay wares, like the famous vitrified 
brick of Staffordshire, the “Staffordshire Blues,” are made by 
thoroughly oxidizing the ware and vitrifying it in the normal 
manner, and then reducing the iron at a high temperature 
(1300°C). This ware is noted for its strength and durability. 

This evidence seems to indicate that the mere existence of 
iron in the ferrous condition in a silicate, or the change from the 
ferric to the ferrous, is not necessarily accompanied with more 
easy fusion, or any signs of poor structure due to over-fire. 


ON TECHNICAL INVESTIGATION. 


75 


Looking at the other side of the problem, why do the black 
cores formed in clay wares so generally show the scoriacious 
structure, and swell long before the red portions, unless from 
greater fusibility? The explanation suggested at this point is 
that the difference of temperature of fusion, if any exists, is not 
the controlling or predominating influence, and that the real 
factor is the relative volumes of gas which the two areas contain. 
It will be shown later in this report, that gases or gasforming 
impurities still occur in the black portions to a very much greater 
degree than in the red or oxidized portions. Supposing an equal 
and fairly complete degree of vitrification obtains in both the 
black area and the red, but that the former is disengaging gases 
rapidly and the latter is not. The red exterior will not swell, 
while tin 1 black interior will sponge up. The apparent vitrifica¬ 
tion of the two areas will be far apart, without any necessary 
difference existing in this respect. 

The Blue-Stoning Reaction. By this term is meant the 
change from lighter to darker colors of red or buff shown by 
clays in passing through their vitrification range into over-fire. 
In some clays the changes are very gradual, and while great in 
total amount are never great between any two contiguous tem¬ 
perature periods. In others, the clay remains in nearly one tint 
for a considerable period of heat treatment, only to change sud¬ 
denly from red to brown or black, or from buff to blue or gray. 
The intimate causes of this change are not known with any 
certainty, but there are two theories: 

Seger holds* that: 

1. This deepening in tint of red burning clay wares with 
increase of temperature is due to increase in density of ferric 
oxide 

2. The increase in density of the ferric oxide is augmented 
by an increase in density of the ware itself, and especially by the 
closing of the pores upon vitrification. 

3. The color will be influenced by the chemical condition 
of the ferric oxide; i. e., whether it is mixed with the clay me¬ 
chanically, or is in combination with some of the constituents. 
Glasses and slags with a high content of ferric oxide have a 
brown color, and show tints such as free oxide never assumes. 

^Collected Writings, p. 107. 



76 


THE N. B. M. A. COMMITTEE 


It has been held by others* that the marked deepening of 
color after vitrification was due to the breaking down of ferric 
compounds and the formation of ferrous silicate. 

Considering the data at our disposition in this test, we find 
that in the exteriors of Burn A and in Burn B, both burns having 
been completed under oxidizing conditions, that there was a 
gradual darkening of color as the temperature rose. Starting 
from light salmon the color passed through pink and light red 
as vitrification temperatures were approached. When vitrifica¬ 
tion took place there was a perceptible deepening of red color, 
and as the temperature was carried still higher this color deep¬ 
ened into a dark red with a touch of brown in it. 

These changes of color were accompanied by some chemical 
changes as well. The proportion of ferrous iron stood as shown 
in table 4, on page 63. A gradual increase in amount of ferrous 
oxide, with a gradual and proportionate decrease of ferric oxide 
undeniably'took place, and would tend, on the surface, to support 
the second view given. 

On the other hand, the changes in color were not coincident 
or coordinate with the changes in the status of the iron. No. 9, 
with 15-16 percent of its iron in the ferrous state, was much 
lighter in color than Nos 28 and 29, containing 9.91 and 11.84 
percent of their iron respectively in the ferrous state. No. 28, 
with 9.91% of its iron in the ferrous state was much darker than 
No. 25 with 10.35%. 

From the preceding experiments, we may say that the second 
theory is thus doubtfully supported by the evidence. The dark¬ 
ening in color seems to a great extent independent of the forma¬ 
tion of ferrous oxide in the samples, and thus the theory of Seger 
is proportionately strengthened. The most that can be said for 
the ferrous oxide theory is that it might be a contributary cause 
without violence to the data found, but that it is not proven to 
have any vital connection with the phenomenon. 

The Over-Fire reaction. The theory has been advanced that 
the puffing of properly oxidized clay ware on over-heating is 
caused by the breaking down of ferric oxide to form a ferrous 
silicate, with evolution of oxygen, the oxygen being supposed to 
cause the puffing.f 


*Orton, Trans. A. 0. S., Vol. V, page 424. 
t.Tackson, T. E. C. S., 1903-1904, page 43. 



ON TECHNICAL INVESTIGATION. 


77 


Segert says, “I have often had opportunity to melt colorless 
glasses together with iron oxide without any other coloring agent. 
When the glass was clear, free from sulphuric acid, which is 
usually not the case, and the ferric oxide was purified from sul¬ 
phuric acid, it was found that the oxide dissolved without any 
evolution of gas, and in oxidizing conditions produced a yellow 
to red brown color. I can therefore not yield the point that ferric 
oxide dissolves in the glass to form ferric-ferrous oxide with an 
evolution of oxygen.” 

The fusible fatrix of a vitrifying brick is fairly comparable 
to a porcelain glaze batch. In both cases, we have a fusion of 
silica and approximately the same bases. If ferric oxide acts in 
a certain way upon going into solution in a glaze batch, why 
should it not act in the same way on going into solution in the 
matrix of a brick? Some easily fusible red burning clays are 
widely used as glazes under the name of “slips.” 

When our thoroughly oxidized samples went into vitrifica¬ 
tion and swelled as in No. 25 to No. 30, there was no notable 
.decrease in the amount of ferric oxide. The small increase in the 
amount of ferrous oxide might easily have been due to the re¬ 
ducing conditions incident to raising the heat rapidly, or to the 
action of gases coming out from the core and affecting the iron 
as they passed through. The samples went into viscous fusion 
and vet the bulk of the iron stayed in the ferric condition. 

Geo. C. Matson* burned four different clays, and also a 
mixture of magnetite and feldspar to viscosity. He found that 
in every case the amount of ferrous iron in the viscous mass was 
much smaller than it was in the raw clay. It was also smaller 
than it was in the same clay burned only to a porous condition 
and not vitrified. 

ITopwood** notes that Sidot has shown that ferric oxide is 
changed to magnetic oxide by prolonged ignition at white heat, 
but goes on to say that “we can not infer from this that the 
ferric oxide in clays suffers such a decomposition when heated 
in clay ware kilns.” He further states that he found no mag¬ 
netic—that is, ferroso-ferric—oxide in tiles heated under oxidiz- 


fCollected Writings of Seger, page 1037. 

*€lay-Worker, XLII, No. 2, page 44. Also reproduced on page 19, 
this report. 

**Trans. Eng. Cer. Soc., 190’3-1904, page 40. 



78 


THE N. B. M. A. COMMITTEE 


ing conditions in a china biscuit oven, at a temperature of 
1350°C. Hopwood and Jacksonf fired ferruginous clay wares in 
an oxidizing atmosphere at a temperature of 1700 C without 
noticing any bloating. 

Seger ascribes the yellow color of porcelain to ferric oxide 
and says! that during oxidation, porcelain will always be yellow. 
German porcelain is fired at a far higher temperature than red 
brick clavs. 

If the vesicular structure of clay ware was due to the simple 
overheating of ferric oxide, then this phenomenon ought to show 
itself at approximately the same temperature in all cases, but 
we find that clays break down and become spongy anywhere 
from 1100°C to about 1700°C. 

In the tests made in this investigation, Burn A gives but 
little evidence. The ferrous oxide constituted 5.81, 6.15 and 9.50 
percent of the total iron in Nos. 1, 1 and 7 respectively. During 
these changes the samples had changed from granular to vitrified 
red bodies but without swelling. In the next sample, No. 9, the 
ferrous oxide jumps up suddenly from 9.50 to 15.2-16.3, and at 
the same time the oxidized exterior of the ware swells and darkens 
in color. 

Tn Burn B, the results are more valuable, because the sam¬ 
ples cover the period more fully. The figures run: 


TABLE No. 7. 


No 

Texture 

Percentage of Iron in 
Ferrous condition 

21 

Vesicular spongy mass . 

8.57— 8.54 

22 

Vitrified and beginning to swell. 

10.03— 9.91 

25 

28 

Vitrified and dense . 

9 91 — 10 35 

Granular and harder . 

9.91— 9.91 

29 

Granular and porous . 

11.36—11.84 


The proportion of ferrous oxide increases as the brickettes 
become harder, but changes in hardness are not accompanied by 
commensurate changes in status of the iron, as in numbers 25 and 

fTrans. Eng. Cer. 'Soc., 1901-1902, page 110. 

^Collected Writings, page 1038. 


















ON TECHNICAL INVESTIGATION. 


79 


28, for instance. Number 28 and 29 show a change in the right 
direction, but it is by no means proven that the cause of the 
increase in FeO was by breaking down of Fe 2 0 3 . 

Burn C has no evidence on the point at issue. 

The writers have had experience with a common brick clay, 
probably of alluvial origin, which passed from vitrification into 
fusion without undergoing the usual vesicular stage. It reached 
a sufficient degree of fluidity to flow down between two other 
bricks, where it chilled in a compact glassy mass, of brilliant 
black luster. This clay must undoubtedly have contained iron — 
probably a good deal of it—and presumably this iron had passed 
from the ferric to the ferrous state in part at least during the 
burning process, as the black color of the glass surely points to 
ferrous oxide. However, the mass did not suggest the slightest 
sign of vesicular structure, as it should have done according to 
the dissociation theory. 

The figures obtained in this investigation do not go any great 
distance towards establishing the dissociation theory, and in the 
face of the last cited example and the weight of opinion adduced 
on the other side, make the further support of this theory at this 
time untenable. 


Summary. A review of the evidence on the action of ferrous 
and ferric oxides in the vitrification of clay brings us to the fol¬ 
lowing conclusions: 

1st. The clay under discussion forms a black core normally, 
but seems abnormal in the fact that it also fuses in the red oxi¬ 
dized portions at about the same time, and both portions become 
nearly equally vesicular. 

2nd. The same clay, after undergoing proper oxidation and 
having developed a dense red vitrified body, will form a blue 
vitrified body by reduction, which will swell from further heating 
at about the same time as the red unreduced portion. 

3rd. Neither the red nor blue vitrified portions in either 
center or exterior swell until they have passed through a dense 
strong solid phase. 

4th. Normal bricks studied in other instances seem to show 
a much wider temperature interval between the swelling of the 
black core and the swelling of the red portion from overfire. 

5th. Evolution of gases in the vitrified mass is the ack- 


80 


THE N. B. M. A. COMMITTEE 


nowledged cause of swelling, whether it be in red or black 
portions. 

6th. Variation in the amount of gas-forming matter is 
suggested as the probable cause of the great apparent difference 
in the fusibility of the black and red portions, the black portion 
with much gas in it appearing more fusible than the red portion 
which still remains solid and unswollen. 

7th. The question as to the difference in melting point of 
ferrous vs, ferric silicates is not settled by this evidence. Indica¬ 
tions point to there being some difference, but a much smaller one 
than indicated by Wheeler; and that this difference is not the 
fundamental cause of the black coring reaction. 

8th. The color changes in this clay do not strongly support 
the theory that the blue stoning reaction is caused by the increase 
in the proportion of ferrous oxide at the expense of the ferric 
oxide, and hence do not tend to weaken Seger’s theory of the 
change of color by condensation of the volume of the iron. 

9th. The figures obtained in this investigation do not 
strongly support the theory that the swelling during over-fire 
is occasioned by the dissociation of ferric oxide into ferrous oxide 
and oxygen, and much other evidence can be marshalled against 
this theory. 

10th. The evidence altogether casts grave doubt on the state 
of oxidation of the iron being the cause of the black coring or 
swelling reaction. 


SULPHUR. 


Sulphur may be present in a burnt clay as free sulphides of 
the metals like iron, or as such sulphides in solution in silicate 
slags, or as sulphates of the alkalies, earths or common meta’s. 
The sulphides are not soluble in water, either when free or in 
solution in slags, but are usually easily dissolved by acids. The 
sulphates are easily soluble in water. Both of these forms, soluble 
and insoluble were determined, the latter by difference between 
the soluble and total sulphur. 

Methods of Analysis. Total Sulphur. This was determined 
in the usual gravimetric way. A one gram sample was fused 
with dry sodium carbonate over an alcohol lamp, disintegrated in 
water, acidified with hydrochloric acid, evaporated to dryness and 
taken up with acid and water. The silica was filtered off, and to 


ON TECHNICAL INVESTIGATION. 


81 


the filtrate was added an excess of barium chloride. The precipi¬ 
tate was allowed to settle over night, was filtered off and washed 
first with dilute hydrochloric acid, and then with water containing 
a little ethyl alcohol. The filter paper was then charred and 
slowly consumed, and the precipitate finally ignited over a bunsen 
flame only when carbon had been pretty well burnt out. The 
crucible was then cooled, a couple of drops of sulphuric acid were 
added, and the crucible again heated till fumes of sulphuric acid 
ceased to come off. The precipitate was then weighed as barium 
sulphate and the sulphur calculated. 

An alcohol lamp employed in fusing these burnt samples of 
clay was employed because alcohol is the only reasonably cheap 
form of sulphur-free fuel available in the laboratory. All artifi¬ 
cial gases and natural gas contain it, and a sodium carbonate 
fusion cannot be made over a flame impregnated with sulphur, 
without some of it being taken up and thus vitiating the results. 
Sulphur-free alcohol is easily obtained. The construction of an 
alcohol lamp which could generate heat enough to make a hard 
fusion was a problem which had to be solved before we could 
proceed. The following device was found very convenient and 
entirely successful, although it is rather cumbersome and awk¬ 
ward in appearance: 


82 


THE N. B. M. A. COMMITTEE 



Fig. 24. Adjustable alcohol lamp for use with blast, in making 
fusions for sulphur analyses. 






































ON TECHNICAL INVESTIGATION. 


83 


The reservoir of the lamp A is a piece of gaspipe 1 y 2 inches 
in diameter by one foot long. It is capped at B, and at C is cov¬ 
ered with a reducer which brings it down to half inch pipe. 
The tube through which the wick is inserted, D, is made of a 
one-half inch gas pipe nipple and elbow. These fittings are cheap 
and everywhere obtainable, and cannot be damaged by heating 
or upsetting or any other ordinary accidents. The wick itself is 
long and of large volume but very soft and unconsolidated, so as 
to permit very rapid volatilization of the alcohol. The wick runs 
back through C and extends well into A. 

The lamp is mounted on an ordinary iron retort stand, held 
by a pair of large burette or apparatus clamps, of the double 
swivelled variety, which can be raised and lowered, moved in and 
out, and twisted on the horizontal axis as well. This clamp per¬ 
mits the lamp to be instantly adjusted in any position with regard 
to the blast apparatus E : The twisting motion is the most im¬ 
portant, as it permits the lamp to be tilted so as to bring the 
alcohol to the wick at any rate desired. By tilting C up and B 
down around the axis F as a center, the alcohol must climb the 
wick for a long distance, and will hence support only a small 
fiame. By tilting C down and B up, the alcohol can be brought 
to D under an actual pressure and the flame increased to any 
needed extent. 

The $bove point is the only one of special convenience or 
value. Ordinary alcohol lamps will not give anywhere near heat 
enough for this fusion work, and this device made the fusion as 
easy as with gas. The blast lamp is used with air alone, and the 
nozzle placed almost touching the wick of the lamp and slightly 
above it. By this means, the flame is gathered up into a powerful 
jet or blowpipe flame. 

In this sulphur work, il was necessary to exercise the strictest 
supervision of tin* chemicals used, because sulphates are the 
commonest impurities in chemical reagents. Blanks were run 
on everything, and every process wa‘s checked with accompanying 
blank determinations and the sulphur obtained from the water, 
air, and reagents was duly allowed for in the results. It was not, 
possible to keep all sulphur out of the reagents, but they were 
kept as low as possible. 

Sulphur existing in form of Soluble Salts. Two to five grains 
of the powdered sample were taken and heated, but not boiled, in 


84 


THE N. B. M. A. COMMITTEE 


water made barely acid with hydrochloric acid. The water was 
decanted off into a calibrated flask at intervals of one to two 
hours, and replaced by fresh, until the last decantation gave no 
test for sulphates with barium chloride. The collected decanta¬ 
tions were then diluted by filling the flask up to its calibration 
mark, arid allowed to stand 12 to 18 hours. Then, by means of a 
pipette, an amount of solution corresponding to one gram of the 
original sample was removed. One or two drops of ferric chloride 
were added, then enough ammonium hydroxide to precipitate 
the floculent ferric hydroxide. The iron carried the clay and fine 
mineral dust which had been carried along with it into the solu¬ 
tion, down into the precipitate, which was easily filtered off. The 
perfectly clear filtrate was made acid with hydrochloric acid, and 
the sulphates precipitated as barium sulphate by an excess of 
barium chloride. The precipitate was allowed to settle and was 
ignited and weighed as in the case of the total sulphur. 

Insoluble Sulphur. The difference between the total sulphur 
and that existing in soluble form is counted as insoluble sulphur. 

The Results. The following table, No. 5, gives the results of 
the sulphur tests: 


ON TECHNICAL INVESTIGATION 


85 


Table No. 8. Results of Sulphur Determinations. 


Designation of Burn. 

Number of Draw-Trial. 

Red Portion of Brickettes. 


Black Portion of Brickettes. 

Total amount of 
Sulphur expressed 
as the element. 

Amount of Sulphur 
existing as soluble 
Sulphates, ex¬ 
pressed as the 
element. 

Amount ot Sulphur 

existing as insolu¬ 

ble Sulphide, etc., 
expressed as the 

element. 

Total amount of 

Sulphur, expressed 

as the element. 

Amount of Sulphur 

existing as soluble 

Sulphates, ex¬ 

pressed as ttie 
element. 

Amount of Sulphur 

existing as insolu¬ 

ble Sulphides, etc., 
expressed as the 

element. 

Raw 

Clay 

8.000 

2.995 

0.740 

0.680 

2.260 

2.210 

' Calculated on basis of clay 
' after dehydration loss of 
( 11.16 percent. 



0.910 

0.835 

0.105 

2.40 

0.210 

2.190 


1 

0.950 

0.842 

0.108 


2.39 

0.160 

2 230 



0.680 

0.240 

0.440 

2.28 

0.200 

2.080 


4 

0.670 

0.210 

0.460 

2.25 

0.195 

2.055 

< 








d 


0.630 

0.096 

0.434 

2 15 

0.164 

1.986 

0 

7 

0.520 

0.096 

0.424 

2 12 

0.157 

1.963 

PQ 










0.320 

0.068 

0.257 

2.06 

0.057 

2.003 


9 

0.310 

0.065 

0.255 

2.04 

0.051 

1.989 



No red 

portion 

produced 

1 84 

0.011 

1.329 


14 

in 

this 

sample 

1 30 

0.009 

1 291 



0.290 

0.051 

0.239 

■> 





21 

0.280 

0.051 

0.229 







0.270 

0.050 

0.220 






22 

0.260 

0.044 

0.216 





PQ 


0.234 

0.045 

0.189 


^These samples had no black 

W 

a 

25 

0.217 

0.044 

0.173 


core or black exterior. 

pq 










0.220 

0.027 

0.193 






28 

0.213 

0.025 

0.188 







0.172 

0.023 

0 149 






29 

0.172 

0.016 

0.166 





s 


0.213 

0.030 

0.183 

0.266 

0.030 

0.236 

d o 

pq 

87 

0.207 

1 

0.030 

0 177 

0 255 

0 016 

0.239 







































































































































86 


THE N. B M A COMMITTEE 


Discussion of the Results. The Determinations themselves. 
The analytical results speak for themselves. The checks between 
duplicates average exceedingly close, and in no instance is there 
any wide discrepancy. The duplicates, and the orderly sequence 
of the changes shown between successive brickettes, all goes to 
prove that the work is accurate, or at least if any error exists it 
is a continuous error which does not vitiate the results for com¬ 
parison purposes. 

The absolute amount of sulphur present is very high—one 
of the worst cases which has come to our notice. Ordinary clay 
analyses do not report sulphur at all, but in the cases where it is 
determined, the amounts usually fall below one percent of sul¬ 
phuric acid (SO s ) or 0.40 sulphur (S). 

The Red Portion of Brickettes. Observing this portion of 


the table, we see: 

1st. That the sulphur experiences rapid expulsion in its 
total amount. At draw trial No. 1, taken at 710° after 17 hours, 
its quantity was only about 30% of the initial. At draw trial 
No. 9, taken at 1100° after 48 hours, the sulphur was only 10% 
of the initial. From here on, the losses dwindle and become very 
small. For instance, in Burn B, test piece 21, taken at 74 hours 
at 1060°C, the sulphur had come almost to the same point that it 
had reached in Burn A in 48 hours, and the losses in Burn B, up 
to the point where the clay became a spongy mass, floating on 
water, were very trifling. In this case, then, we may say that the 
loss of sulphur goes on under oxidizing conditions steadily, either 
with rising temperatures or with stationary temperatures, until 
the total residual sulphur is about 10% of the initial, after which 
its changes are slow and gradual. 

2nd. The soluble sulphur, existing as sulphates if lime and 
other bases are first increased a little in the early stages of burn¬ 
ing. Thus the soluble sulphur in the clay was 0.71 average, and 
0.84 in the draw trial taken at 710°C (17 hours). The reason is 
not far to seek. The sulphides naturally oxidize to sulphates 
when treated with pure hot air, or if sulphur is expelled from 
FeS 2 by dissociation at 400°C and the sulphur takes fire and 
burns to S0 2 or S0 3 , it is clear that it is held in the clay in part, 
by new combinations formed there. 

3rd. The period when soluble sulphates are formed and 
retained unaltered in the clay is short, however, for at draw trial 


ON TECHNICAL INVESTIGATION. 


87 


4, taken at 805 C, 31 hours, there had been a heavy loss of soluble 
sulphur. Evidently the sulphates or sulphites formed are not 
very stubborn combinations, or they would not break down so 
easily. The soluble sulphates found from this point to the end 
of ih e third burn become less and less, and goes to show that the 
best method to overcome this salt in bricks and other architectural 
products is simply to heat them up to a point of moderate vitrifi¬ 
cation and hold them there. 

4th. The insoluble sulphur, comprising sulphides and siiica- 
sulphur compounds, undergoes some peculiar changes in the early 
part of the burn. Evidently, the heat absorbed below 700°C has 
powerfully affected the original combinations, for of 2.25% of 
insoluble sulphides in the clay, only 0.10% remains. Evidently, 
a large proportion of the sulphides are not only broken down 
but expelled also. A little is saved—enough to increase the sol¬ 
uble sulphates a little, but not much. But, after passing 700°, 
the conditions are reversed. The sulphur remaining at that time 
is mainly soluble in water, but heating to 800° C renders it again 
insoluble. 

Thus, the 0.10% at 700°C becomes 0.44% at 800°C, and part 
of this must have been derived from the 0.84% soluble sulphur 
at 700°C. 

From 800°C on, this residual sulphur hangs on doggedly, 
and is expelled only a little at a time, and apparently time is more 
important than temperature in getting rid of it. Nearly all of 
the residual sulphur from 800°C is of the insoluble form. It 
would be interesting to know what the form is, but we have only 
hypotheses to offer. It may be sulphides held in silicate solutions, 
like the matter produced in smelting many ores. There is no 
question of the ability of silicate slags to dissolve and hold 
sulphides in themselves, or at least to disguise their presence so 
that they are not detected by the eye as a foreign ingredient. 
There is also no question but that sulphates, or sulphuric acid, is 
soluble in silicate slags and glasses. Either of these forms would 
hang on to their sulphur well, and probably would not give them 
up without the interference of some new reaction—other than 
air oxidation. 

These changes in the sulphur content are well shown in the 
following curves: 


88 


THE N. B. M. A. COMMITTEE 



Temperatures m Degrees Centigrade 
Fig. 25. 













































ON TECHNICAL INVESTIGATION. 


89 


The preceding curve is drawn in such a way that the course 
of the determinations is shown but that, if the whole area of the 
curve sheet be considered to represent all of the sulphur of the 
clay during the whole time of the test, then 

The area A. A. A. represents the quantity expelled. 

The area B. B. represents the insoluble sulphur remaining. 

The area C. C. represents the soluble sulphur remaining. 

The areas B and C together represent the total sulphur re¬ 
maining. 

These areas show beautifully by their size and shape the 
various events and situations, especially the increase of the soluble 
sulphates for a time and their succeeding rapid diminution. 

The Black Portion of the Brickettes. Observing this portion 
of Table No. 5, we find a very different state of affairs: 

1st. The total sulphur drops away slowly and reluctantly, 
having at the advanced stage of brickette No. 9 (48 hours. 
1100°C) only lost about 30% of its initial quantity. When 
pushed up to strong vesicular fusion, the rate of loss increases, 
but even then nearly one half of the sulphur remains. 

2nd. The soluble sulphur follows the same general course 
in the black portion as in the red, with the important exception 
that it shows no marked increase in the lowest temperature 
(700°C) trial piece. This indicates that the cause of the marked 
increase of soluble sulphur in the red exterior portion is due to 
the oxidation of the sulphides to sulphates, while in the black in¬ 
terior, oxidation of the sulphur would not be likely to occur while 
carbon was present and still unoxidized. Thus in draw trial 
No. 1, we have 0.84% soluble sulphur on the outside, or more 
than the raw clay contained, while we have 0.185 on the inside; 
in the outside, we have 88% of the sulphur present, in the soluble 
form, while in the inside we have 2.5 times as much sulphur 
present, and of this only 7.7% is soluble. 

These figures sufficiently prove the entire reversal in ten¬ 
dency brought about by the presence of the carbon. It seems not 
only to have delayed the expulsion of the sulphur from the clay, 
but it seems to have reduced to some extent the sulphates back to 
insoluble sulphides. After the heat begins to rise (800° and 
above) the soluble sulphur is reduced steadily to vanishing quan¬ 
tities, very much as in the exterior, though proceeding a little 
faster all the way along. 


90 


THE N. B. M. A. COMMITTEE 


3rd. As a natural corrolary to what has preceded in 1 and 
2, the insoluble sulphur in the black portions remain high 
throughout—nearly up to the total. This simply emphasizes the 
fact that the sulphides in the center are protected from oxidation, 
both by their location and by the surrounding carbon, and this 
enables them to stand comparatively high temperatures with but 
little loss. 

4th. When the clay has been pretty completely oxidized 
and the sulphur expelled by normal processes, the application of 
a heavy reducing atmosphere, which undoubtedly contains some 
sulphur, may cause re-absorption of sulphur. In this case, there 
was a small but clearly marked increase in the exterior of the 
ware. Thus the blue black exterior of sample 37 contains 0.266, 
while the red and unreduced interior contains only 0.210. 

The relationships between the total and soluble sulphur in 
the black cores are well shown in the following curve: 


ON TECHNICAL INVESTIGATION. 


91 



o 

o 

N 


o 

o 


o 

o 

o 


o 

o 

<x> 


o 

o 

© 


o 

o 

s 


o 

o 


O 

O 

<0 


§ 

M 


O 

o 


Temperatures /n Degrees Cer?f7grade 
Figure 26. 












































92 


THE N. B. M. A. COMMITTEE 


This curve is drawn so that it divides the field up into areas 
proportional to the amounts of sulphur in the various forms dur¬ 
ing the progress of the test. 

The area A. A. represents the amounts of sulphur expelled 
from the center of the clay in burning, from the beginning up to 
vesicular fusion and the formation of a sponge. 

The area C. C. represents the amount of sulphur existing in 
the soluble condition during the range of the test. 

The large area B. B. B. indicates the amount of sulphur 
which is held in insoluble form in the clay through the action of 
the carbon in the core. 

The Reactions by which Sulphur is Expelled. There are 
probably a number of ways in which sulphur may be expelled 
during the burning process. Only some of the simpler ones, of 
whose operation there can be little doubt, will be given. 

The Normal breaking down of Pyrite. Pyrites, when heated, 
suffers decomposition, as expressed by the reaction 

FeS, at 400°C=FeS+S. 

The free molecule of sulphur distills off as such, as can easily 
be verified by conducting the operation in a closed glass tube, 
where it can be kept free from air. When conducted in the open 
air, the S immediately catches fire and burns to S0 2 or S0 3 in¬ 
stead of depositing as a coating or crystalline fibre on the sides 
of the glass. When it occurs in a clay, at some distance beneath 
the surface, the action resembles, to some extent, that of the 
closed glass tube, in that air finds trouble in reaching it at once. 
But the conditions differ from those in the glass tube, in that the 
surrounding body is spongy, and allows gases under pressure to 
make their way out in all directions, and the clay being hotter as 
the surface is approached, there is little or no likelihood of the 
sublimate recondensing on its outward journey. 

The outgoing sulphur fumes are very apt therefore to pass 
over the bases, some of which have been newly relieved from C0 2 , 
H 2 0, etc., and are ready for or susceptible to attack by the 
sulphur. Iron (FeO), lime (CaO) and magnesia (MgO) all a.re 
liable to this kind of attack. 

There can be no doubt, however, that the bulk of the sulphur 
from FeS 2 broken up in the clay, escapes with the waste gases 
and the clay is relieved of it. 

2nd. The Slow Oxidation Process. After FeS 2 has broken 


ON TECHNICAL INVESTIGATION. 


93 


down, and lost one half of its sulphur, the remaining FeS would 
oxidize in the air, if given a chance, at temperatures of 400°C 
and above. But this condition seldom lasts long, for the clay 
usually is pushed rapidly along to incipient vitrification, and its 
air supply grows steadily less as its pores contract and become 
choked and discontinuous. Thus in figure 25, the course of the 
total sulphur curve from 0 to 700°C illustrates the operation of 
the first reaction, or the breaking down of the pyrites, beginning 
at about 400° and losing 70% of the sulphur in the next 300°. 
The second reaction, or the roasting out of the sulphur from the 
FeS is illustrated by the curve from 700 to 1100. Here the rate 
of loss is slow and fairly regular, and it is shown that mere heat¬ 
ing up of the clay in oxidizing atmosphere is not sufficient to 
drive out all of this residual sulphur in any reasonable time. 
Further, the removal of sulphur from FeS cannot be accom¬ 
plished by heat alone, for the substance is readily fusible, at about 
1000-1100°G, and when fused is readily taken into solution in 
silicate combinations. 

3rd. The Dissociation of Sulphates. Ferrous sulphate in 
oxidizing atmosphere loses its sulphuric acid (SO :i ) rapidly and 
fairly completely at a temperature of dull redness—550-650°C. 
The product, the ferrous oxide, promptly changes to ferric oxide, 
producing the material familiarly known in the paint trade as 
Venetian Red. The color of the oxide varies with the temperature 
at which the decomposition takes place. It is brightest red at 
the lowest temperature, and grows darker, till it becomes purple 
and almost violet-black at high temperatures. The last 2 per cent 
or so of sulphuric acid sticks obstinately, and cannot be expelled 
without heating the material so high as to spoil its bright red 
color. Hence, commercial Venetian red is ordinarily impregnated 
with sulphate of iron, which causes it to make trouble when used 
as a mortar color. 

Calcium sulphate also breaks down, but at considerably 
higher temperatures than ferrous sulphate, and less completely. 

It is probable that the intervention of reducing conditions 
are necessary to effect a satisfactory decomposition. No case of 
CaS0 4 , breaking down with evolution if visible SO ;! has come 
under the writer’s observation, but cases of breaking down with 
copious evolution of S0 2 have been witnessed, and under condi¬ 
tions which made the explanation of the source of the reducing 


94 


THE N. B. M. A. COMMITTEE 


agent or action very obscure. The use of CaS0 4 in porcelain and 
Na. 2 S0 4 in the batch for glass manufacture is always in connection 
with reduction as a means of expelling the sulphur. 

On observing the curve sheet, Fig. 25, it is seen that the 
sulphates, represented by the curve of the soluble sulphur, in¬ 
crease in amount up to 700°C. While the sulphur as a whole has 
decreased 70%, the sulphates have increased 16%. Immediately 
after 700°C, however, the very abrupt fall in the line shows that 
some of the sulphates are decomposing—probably the FeS0 4 . 

About 75% of the sulphates are broken up and expelled 
during the next 100°C. The remaining 25%, probably compris¬ 
ing the calcium sulphate chiefly, are expelled steadily but slowly, 
so that a small amount remains still after the clay has fused to a 


sponge. 

Influence of Carbon and Iron. It appears from a study of 
Figure 26, that the central portion of the clay gave up its total 
carbon very slowly, and that the proportion of sulphates was not 
increased by oxidation in the early part of the burn as in the 
outside parts, and that the bulk of the sulphur in the clay re¬ 
mained there, in insoluble form, until about 1100°C. 

Since these events are so diametrically opposite to what 
occurred in the exterior, or carbon-free portion, and since no 
other known cause of variation between the inner and outer por¬ 
tions existed, we are compelled to conclude that, the carbon has 
been responsible for this change. The mode of its action is not 
known and can only be conjectured. 

In order to bring these relationships more clearly into view, 
the following curve sheet, Figure No. 27, has been prepared: 


ON TECHNICAL INVESTIGATION. 


95 





iperafures in Degrees Centigrade 
Figure 27. 















































96 


THE N. B. M. A. COMMITTEE 


From tliis figure, it is seen 

1st. That both carbon and interior sulphur have lost 20% 
up to 700°C. That exterior sulphur has lost 68% in the same 
period. 

2nd. That carbon continues to lose rapidly, until all is gone 
at about 1000° C. 

3rd. That at 1000°C the interior sulphur has lost an addi¬ 
tion at 6 or 8%, while the exterior sulphur has lost about 90%. 

4th. That the sulphur of the interior soon after 1000° C 
begins to be expelled at a much more rapid rate, while the rate 
at which the sulphur of the exterior escapes remains unaffected 
by the expulsion of the last of the carbon. 

On the other hand, it must be allowed that the physical 
factors in the case probably are exerting some influence, and are 
eo-responsible with the carbon for the late expulsion of the 
sulphur from the interior. The last of the carbon disappeared 
at about 1000°C, but the rate at which the sulphur was expelled 
did not at once change. Doubtless, the difficulty of oxygen getting 
at it is responsible for this fact, for the clay at 1000° C is so 
dense that gases could only get in or out slowly and wit|h 
difficulty. 

It may also be asked, how does the carbon act upon the sul¬ 
phur compounds to restrain their expulsion. The most natural 
hypothesis is that when FeS 2 is broken down into FeS and S, the 
free sulphur instead of being expelled, as is the case in the 
exterior, combines with the ferrous oxide present, forming FeS. 
The iron, as shown on the curves (see figures 22 and 23) begins 
with 62% in the ferrous form, probably mostly ferrous carbo¬ 
nate. From results of an incomplete test, made in 1902, # we 
know that ferrous carbonate breaks down and gives up its CCh 
as low as 425°C, by the reaction: FeC0 3 +heat==FeO+C(X. 
If meanwhile, free sulphur is liberated in its immediate vicinity, 
and in the presence of carbon, the reaction 

FeO+C+S=FeS+CO 

would be likely to occur. The probability of this is increased 
from the study of the curves Fig. 27, where the carbon is rapid'y 
expelled, while the ferrous iron increases and the sulphur remains 
almost in statu quo. 

Inasmuch as the chart, Fig. 27, is constructed so as to show 


♦Trans. Am. Cer. Soc., Vol. V, p. 400. 



ON TECHNICAL INVESTIGATION. 


97 


the three reagents as percentages of their initial quantities, the 
point naturally arises whether the actual quantities present are 
sufficient to make the above reaction quantitatively possible. A 
calculation shows that they are; even after allowing for the losses 
of carbon at various points in process, there is always enough left 
to more than accomplish the dioxidation of the iron as shown. 

Thus a logical and entirely plausible explanation of the slow 
removal of the sulphur from the dark core or interior portion of 
the brickettes is shown in the conversion of the sulphur into the 
form of ferrous sulphide, FeS, by the joint cooperation of the 
ferrous oxide and free carbon. As has already been explained, 
ferrous sulphide once formed can not be broken up by mere 
heat. It must be either decomposed by roasting in air, or by the 
reactions to be next described. 

Expulsion of Sulphur by Silicic Acid. It has long been 
known that at high temperatures silicic acid becomes a very 
active acid, displacing all other common acids, and combining 
wtli their bases to form silicates. In this way it has the power to 
replace sulphuric acid, and the sulphur of sulphides. 

Seger* fund that a bisilicate glass mixture, saturated with 
sulphates, showed a sulphuric acid content of 4%, while the same 
glass with one extra molecule of silica added and melted in at 
the same temperature and under the same conditions gave a glass 
which showed a sulphuric acid content of only 2%. From this 
he drew the conclusion that a bisilicate mixture saturated with 
sulphuric acid when changed to a trisilicate by taking up silicic 
acid, discharged 2% of its sulphuric acid in the form of sulphur¬ 
ous acid and oxygen. 

It is a fact of great commercial importance in blast furnace 
practice that a basic slag, that is, one low in silica, will absorb 
sulphur, but that an acid slag, that is, one high in silica., will give 
off sulphur. 

These two samples go to show that as long as a fused silicate 
mixture is quite basic in character, sulphur will not be affected, 
but, as the mixture becomes more silicious, the silicic acid will 
tend to replace either sulphuric acid or the sulphur of a sulphide. 

The first stage in the vitrification of a clay ware is the for¬ 
mation of a fusible matrix in the interstices of a less fusible 
skeleton. We do not know the constitution of this matrix, nor 

^Collected Writings, page 646. 


98 


THE N. B. M. A. COMMITTEE 


is it likely to be the same in different cases, but we have had a 
general idea that it consists in the greater part of the more fusible 
fluxing bases, with a small amount of silica. 

Of late years, scientists have carried on a considerable 
amount of study regarding the properties of fusing slags and 
silicate mixtures, and have given a new name to the most fusible 
combination possible at any given temperature with a given set 
of ingredients. Thev call it a ‘"Eutectic.” The action of the 
eutectic is selective; that is, the various ingredients in a mixture 
do not go into solution in the proportion in which they are 
present, but in the proportion in which they are needed to make 
the most fusible mixture possible. It has been found that the 
eutectic is always very basic at low temperatures, and becomes 
more acid as the temperature rises. 

Keeping the two statements in mind: first, that as a silicate 
fusion becomes more silicious it will drive off sulphur; second, 
that the fused matrix of a clay is a basic silicate mixture, gradu¬ 
ally becoming more silicious as the temperature rises, we can 
easily understand the swelling and marked vesicular structure 
developed in our samples. 

(a) Sulphur was present in the samples, as we know from 
the analyses. 

(b) The samples reached vitrification without puffing, as 
is seen by inspection of the trial pieces. 

(c) The fused matrix in these vitrifying brickettes was a 
basic silicate mixture, gradually becoming more silicons—the 
study of eutectics in other connections has shown that this is 
always the case. 

(d) When the matrix became silicious enough, silica would 
have begun to drive off the sulphur; experience with other silicate 
mixtures has shown that it should do so, and the analyses show 
that in this case there was a sudden and marked change in the 
rate of sulphur expulsion between 1100° and 1200° C. Moreover, 
this change of rate is apparent in both exterior and interior parts 
of the brickettes, and therefore seems to be independent of the 
former causes of sulphur elimination. 

The exterior portion lost but little actual weight of sulphur 
between 1100 and 1200°C, but it lost nearly 50% of what there 
was present. The interior portion lost a large amount of sulphur, 


ON TECHNICAL INVESTIGATION. 


99 


amounting to one-third of the amount present, or nearly as much 
as had been lost in the 1000° C preceding. 

(e) The reactions by which the silicic acid ousts the sul¬ 
phuric acid or sulphur from sulphates and sulphides, are not 
definitely known. They may be something like the following: 

In the oxidized portions of the sample where most of the 
sulphur was present as sulphate: 

MS0 4 +Si0 2 =M0Si0 2 +S0 3 
where M is any bivalent base, or 

M 2 (S0 4 ) 3 +Si0 2 =M 2 0 3 Si0 2 d-3 S0 3 
where M is any trivalent base. 

In the black cores which had never been oxidized, and in the 
portions that had first been oxidized and then heavily reduced, as 
the exteriors of draw trials No. 37 to No. 40, the sulphur was in 
all probability in the sulphide form (FeS), and the reaction 
might run like this: 

MS+3 Fe 2 0 3 +7 Si0 2 = MSi0 8 +6 FeO, Si0 2 +S0 2 
where M is a bivalent base, or 

M 2 S 3 +8 Fe 2 0 3 +18 Si0 2 =2 MSi0 3 +18 FeOSi0 2 -f 3 S0 2 
where M is a trivalent base. 

The reactions, when they take place in a brick, are no doubt 
a great deal more complex, but the suggestion given shows the 
fundamental reaction in puffing, namely, the setting free of S0 2 . 

On consulting the curves, Fig. 27, it is seen that in this case, 
while the suggested reactions could easily have taken place in the 
red exterior portions, where Fe 2 0 3 was in abundance, they could 
hardly have done so in the black interior, because all of the iron 
present was already present as ferrous oxide. This is by no 
means a bar to the successful operation of the theory, for it is 
entirely possible that ferrous oxide might be reduced still further 
to Fe, by sulphur disengaged at this high temperature. It is also 
possible that other oxides may give up oxygen under the pressure 
of the increasing activity of the silicic acid. 

(f) The rapidity of the evolution of the gas S0 2 in both 
interior and exterior portions was sufficient to cause the hitherto 
apparently sound structure to become highly spongy—in fact, so 
light as to float on water. No other gases of which we have 
knowledge were disengaged at this time. Water was long ago 
eliminated. Carbon was all expelled at 1000°C and the clays 


LOF C. 


100 


THE N. B. M. A. COMMITTEE 


had not yet swollen markedly, so that it seems that carbonic acid 
or carbonic oxide could scarcely be the actuating cause. 

(g) The volume of the sulphur dioxide (S0 2 ) given off is 
easily able to account for the sudden and extraordinary swelling 
shown by this clay, as is shown by the following computation: 

Taking samples 25 and 29, from the exterior portion of the 
brickettes which cover the observed period of swelling, and ex¬ 
amining their sulphur contents, we find: 

No. 25. 0.0023 gram sulphur in 1 gram sample 

No. 29. 0.0017 gram sulphur in 1 gram sample 

0.0006 gram sulphur lost during swelling 

1500.0000 grams is the weight of the brickette, and assuming 
that the whole of it was affected by the swelling, we would have 
1500.0000X-0606=0.9 grains of sulphur lost by swelling. 

When S0 2 is formed the reaction is 

s+g 2 =so 2 

or, by weights: 32-(-32=64 

Hence 0.9X2—-1.8 grains S0 2 produced. 

1 liter of S0 2 at 0°C and 760 m.m. pressure weighs 2.785 
grams*. 

1 liter of S0 2 at 1100°C and 760 m.m. pressure, would weigh 
0.5261 grams. 

1.8-^ 0.5261=3.4 liters, volume of S0 2 formed from the oxi¬ 
dation of .0006 grams sulphur at 1100°C. 

1 liter=61 cubic inches. 3.4 liters=207.4 cubic inches. 

The volume of the brickette was 2fX X 3% X 3%=37.5 cubic 
inches. 

207.4-f-37.5=5.5 

That is, the volume of S0 2 that would be generated would be 
5.5 times as large as the whole sample. Even if the pore space 
left in the brick were 10% of its volume, the volume of gas would 
be 55 times as large as the space available. 

As the volume of a gas varies inversely as the pressure, when 
the temperature remains the same, to calculate the pressure that 
would have been exerted by this gas, if it had been held in the 
available space, we use the formula 

Atmos, pr. 14.7: X :: 1: 55=808.5 pounds per sq. in. 


^Smithsonian Physical Tables. 




ON TECHNICAL INVESTIGATION 


101 


Thus we see that a very small actual weight of sulphur may 
we]l cause puffing if it gets oxygen for combustion, or if it is 
forced ont of a solid compound and into a gaseous state at this 
high temperature, and surrounded by a viscous and but slightly 
porous body. 

The Interstitial or Residual (las Theory of Swelling. An¬ 
other explanation of the swelling of clays recently set forth by 
Purdy and Moore* shows that the gases filling the interstitial 
spaces in the gradually vitrifying clay structure, and which are 
cut off from escape by the fusing of the walls of the pore chan¬ 
nels, tend to convert the irregular shaped cavities in which they 
are imprisoned into spherical cells, exactly as a glass blower con¬ 
verts a shapeless mass of glass into a sphere by expanding tin* 
gas in the center by blowing it. The development of a vesicular 
structure from this cause is undoubtedly possible, and in fact 
normally to be expected whenever any silicate mass is brought to 
a condition of viscosity by heat. But there is a marked difference 
between the development of a vesicular structure from this cause, 
and that brought about by the sulphur in the present instance, 
for the latter was so very quick in its development and the spong¬ 
iness produced was so exaggerated, while ordinary bloating from 
over-fire develops gradually from very small beginnings. In fact, 
it was shownf that vesicles were forming in the glassy matrix 
at temperatures far below the point of complete vitrification and 
while the structure as a whole was still porous. The swelling is 
due in this case to the simple change of volume of a gas by heat, 
while the sulphur reaction arises from the sudden conversion of 
a solid into a gas, after the clay has already assumed the vitreous 
structure. 

If the facts and deductions concerning this sulphur silica 
reaction are correct, the question may well arise, why are not 
clay wares more generally swelled from its operation? Clay 
wares of the common sorts, especially those made from shales and 
fireclays, generally contain either sulphides or sulphates or both, 
and in amounts larger than the small quantities dealt with in 
paragraph (g). Iron and carbon also are common impurities. 

*Pyrochemical Properties of Clays, Volume IX, Trans. Am. 
Cer. Soc. 

fibid. 



102 


THE N. B. M. A. COMMITTEE 


To this, it may be answered that the bulk of the shales and 
many of the fireclays do break down with a scoriacious structure 
from over-fire—in not infrequent cases, as badly as the sample 
used in these tests. It requires, however, the combined presence 
of all three impurities, to develop the trouble at its worst. Also, 
in the case of clays containing small quantities of the troublesome 
ingredients, the reduction in quantity of the reagents before 
vitrification takes place may reduce the tendency. Also, clays 
with opener texture, or clays with later melting point, would 
probably evade the difficulty by permitting the gases to escape. 
The thickness of the cross section is also of much importance, as 
a thin cross-section permits oxidation clear through the body 
much more easily. 

Summary. A review of the evidence on the influence of sul¬ 
phur brings us to the following list of conclusions: 

1st. Both sulphates and sulphides experience rapid dimi¬ 
nution by dissociation, in that portion of the burn up to 800° C, 
in those portions of the ware which get air freely. This loss of 
sulphur may amount to two-thirds or three-fourths of the amount 
originally present. 

2nd. Both sulphates and sulphides experience a further 
slow diminution by dissociation or oxidation, beginning at 800°C 
and continuing as long as the clay structure remains porous and 
permeable to air. The loss of sulphur may amount to 90% or 
more of the initial sulphur content at the end of the period, but 
it proceeds increasingly slowly, and probably would never become 
complete. 

3rd. In the interior portions of the clay, where air pene¬ 
trates with difficulty, the reactions of paragraphs 1 and 2 may be 
greatly modified. The loss of sulphur will be much less, and if 
there are bases present with which free sulphur may combine, as 
FeO, CaO, or MgO, there is strong likelihood that the sulphur 
will not be expelled. 

4th. The presence of carbon, even in small quantities, inter¬ 
feres still more strongly with the expulsion of sulphur in any 
form, and the sulphur will not suffer much loss until after the 
carbon has been expelled. In many cases the clay has become too 
dense by that time for oxidation of the sulphur to proceed, so 
that the carbon has virtually prevented its escape. 


t 


ON TECHNICAL INTESTIGATION 


103 


5th. The retention of sulphur in any form and from any 
cause in the mass of the clay is not likely to cause physical dis¬ 
turbances in the clay until a fairly complete degree of vitrifica¬ 
tion lias been reached. 

6th. When a clay readies a dense vitrified condition, it 
proceeds normally, after a longer or shorter interval, to become 
less dense, by reason of the development of multitudes of minute 
vesicles in the viscous body; this process is progressive and in the 
end, the body becomes spongy, weak and worthless. 

7th. The length of this period of dense vitrification is much 
shortened, and in some cases practically abolished by the presence 
of sulphur compounds, which break down and evolve gases co¬ 
piously, producing a prematurely spongy body. 

8th. The cause of this gas evolution is chiefly the dissocia¬ 
tion of sulphides and sulphates by silicic acid, which becomes 
increasingly active as the temperature rises, and appropriates the 
bases formerly combined with the sulphur. 

9th. In clays of low sulphur content, and of favorable 
structure for oxidation, the amount of sulphur left in the clay at 
vitrification is very small. Hence the period of good structure 
is long, the vesicular structure develops slowly, and the clay is 
said to stand over-firing well. 

10th. In clays of high sulphur content, or of dense structure 
unfavorable for oxidation, or of high content of iron and carbon, 
the escape of the sulphur is prevented, the clay has a very narrow 
period of usefulness or none at all, and the vesicular structure 
becomes enormously exaggerated. 

11th. While this premature and exaggerated swelling from 
sulphur may in bad cases occur in well oxidized clays, it is practi¬ 
cally certain to occur where clays containing a partly oxidized 
core are allowed to reach the vitrification period. 

12th. This breaking down of sulphur compounds by silicic 
acid, is the chief or common cause of the premature swelling 
of black cored clays, and the occasional cause of sudden and 
severe swelling of properly oxidized clay wares. 

13tli. The proper way to avoid the effects of sulphur in 
vitrifying clay bodies, is to apply a deliberate and complete oxida¬ 
tion treatment, while the clay remains porous. This will rid the 


104 


THE N. B. M. A. COMMITTEE 


clay of the greater part of the sulphur, and will prevent sudden 
or premature slagging of the clay by ferrous oxide, if it is true 
that ferrous oxide has such a tendency, and will thus avoid, so 
far as possible, the conditions which favor the swelling. Clays 
which still give trouble from swelling after this treatment must 
be regarded as bad clays. 

GENERAL CONCLUSIONS* 

In the foregoing pages, the behavior of each of the three 
prime impurities of a clay, if we may so characterize them, has 
been subjected to as careful a scrutiny as the data itself warrants. 
Certain tendencies or modes of action have been strongly indi¬ 
cated by this data, and these tendencies, while by no means to be 
taken as final truths from the testimony of this one clay—a clay 
which it must be freely admitted is extraordinary—nevertheless 
foreshadow the lines of action which we should expect from the 
same materials in other clays. Until more and better data is at 
hand, therefore, we are justified in provisionally using these 
principles as laws on which to base our future interpretations 
of the composition and resultant properties of clays. Only by 
the verification of these laws by the application of similar meth¬ 
ods to other clays of diverse character, can they assume real or 
lasting acceptance. The following are the deductions which the 
data here presented seems to most strongly support: 

I. The black-coring of clays is a complex process, which 
has its origin and development in the following series of over¬ 
lapping chemical steps or stages: 


Black-Coring. 

(1) Carbon, bituminous matter or organic matter of some 
sort, is a very common, and in fact almost universal component 
of clays. 

(2) On being heated in a clay, it tends to burn out. Owing 
to the physical structure of the clay, the retarding influence of 
water in the clay ware, the retarding influence of kiln atmosphere 
low in oxygen or high in reducing gases, the shortness of time 
allowed, or some other agencies, this burning out is often delayed 
and the clay reaches a strong red heat without being completely 
freed from this material. 


ON TECHNICAL INVESTIGATION. 


105 


(3) When the carbonaceous matter escapes.oxidation until 
temperatures of visible redness in daylight are reached, it tends 
to undergo a destructive distillation, by which a portion is ex¬ 
pelled from the clay in the form of light highly combustible 
gases, which are likely to ignite and burn furiously in the kiln, 
and thus make the situation worse by raising the temperature 
still higher and more rapidly. If the air supply is restricted, by 
closing doors and air intakes, and the gas is allowed to smother 
or only partly burn for lack of oxygen, the temperature may be 
controlled and kept below the danger point of 900°C. 

(4) The residue of the carbon of the clay, called the fixed 
carbon, after the expulsion of the volatile matter, is in the con¬ 
dition of coke, or graphite, or charcoal, deeply ingrained into the 
structure of the clay by the process of distillation. The clay is 
like a sponge permeated by a volatile liquid, which decomposes, 
leaving a residue in every part of the mass. This carbon, being 
non-volatile and more or less dense, cannot be removed except by 
oxidation. Mere heating is not sufficient. 

(5) This carbon may be removed by oxidizing with air, at 
comparatively low temperatures. This is the proper way, but it 
takes time—sometimes a long period—and burners often become 
impatient and proceed without waiting for the safe time. This 
must be done below 900°, while the clay still remains spongy and 
permeable to air going in and the resultant products of oxidation 
going out. If the clay is heated beyond this point, it is apt to 
become so dense that oxidation from outside ceases. 

(6) The carbon may be removed by contact with other 
reducible materials, such as metallic oxides like Fe 2 0 3 and Mn 2 0 3 , 
or acid oxides like S0 3 or N 2 0 5 . In this case, the resultant gases 
are liberated in the clay and strive to make their way out, pro¬ 
bably under pressure. The temperature at which the fixed carbon 
is oxidized b}^ contact with Fe 2 0 3 or S0 3 , etc., is not definitely 
known, but it is not far from 1000° C. At the temperatures at 
which it occurs, few clays if any are so dense as to prohibit the 
escape of the resultant gases. Consequently, there is little likeli¬ 
hood of fixed carbon remaining in a clay until vitrification is 
reached unless the amount is excessive and more than can be 
oxidized by the reduction of any ferric oxide or sulphates present. 

(7) Carbon is a possible but rare actual cause of the dark 
coloring of the core of clays unsufficiently oxidized. It is very 


106 


THE N, B. M. A. COMMITTEE 


doubtful if the escape of the oxidized carbon is a possible actual 
cause of the swelling or vesicular structure of clays. 

(8) The iron found in clays tends in the absence of car¬ 
bon or sulphur to oxidize easily to ferric oxide. In the presence 
of carbon or sulphur, this oxidation is not possible until the bulk 
or whole of these substances have been oxidized. In fact, the 
evidence indicates that the iron oxide itself usually accomplishes 
a part of the oxidation, and is itself left in the magnetic, or fer¬ 
rous, or even spongy metallic condition. 

(9) The iron thus reduced is not likely to be again easily 
oxidized, because, the increased density of the clay constantly 
reduces the circulation of air through its structure. Consequently 
it is highly likely to be taken into silicate combination as vitrifi¬ 
cation proceeds, while still in the lower stages of oxidation, and 
in this case imparts black or dark colors to the clay. 

(10) The view has long been held that the presence of 
iron in lower forms of oxidation than Fe 2 0 3 is a powerful agent 
in slagging the clay at much lower temperatures than if the clay 
contained iron only in the Fe 2 0 3 state. This evidence does not 
support that view, there being but little observable difference in 
fusion temperature of the oxidized and unoxidized portions, 
though the latter were undoubtedly the first to show signs of 
fusion. 


(11) The black coloration of the unoxidized cores of im¬ 
properly fired clay wares is usually chiefly or wholly due to the 
formation of ferrous iron silicates in the clay. 

(12) The formation of ferrous silicate in the cores has not 
been shown to have been accompanied by the evolution of gases 
or the immediate swelling of the ware. Evidence has been 
collected tending to show that perfectly quiet combination be¬ 
tween iron and clay may take place without development of any 
swelling. 

(13) The sulphur of the clay, in whatever form it originally 
occurred, is subjected to partial expulsion by the mere rise in 
temperature of burning, but it cannot be wholly or even princi¬ 
pally expelled by heat alone. By the same oxidizing treatment 
by which the carbon is removed, sulphur is greatly reduced but 
never wholly expelled. Sulphur oxidizes after the carbon how¬ 
ever, and hence where any carbon still remains, the sulphur can 
not be seriously reduced in quantity. 


ON TECHNICAL INVESTIGATION. 


107 


14. Sulphur in any form may give rise to gases at high 
temperatures in the clay, by oxidation or dissociation, or by 
substitution by silicic acid. These reactions are practically cer¬ 
tain to take place in any clay at some point in its vitrification 
range, and when they take place, they cause swelling and vesicu¬ 
lar structure to develop. 

(15) Since the unoxidized portion of the clay contained 
carbon longest, its content of sulphur will be less reduced by the 
action of heat, and its potential capacity to give off gases will be 
greater than the outer or oxidized portion of the clay. This 
difference in the present instance is very great, amounting 'to 
many times as much sulphur. Hence, the amount of gaseous 
matter in the black portion of the ware being so much higher 
than in the red portion, it naturally follows that swelling 
is practically certain to develop first in the black areas. 

We may say then, (a) that carbon is the beginning or first 
cause of the black-coring reaction, by preventing the timely 
oxidation of the sulphur and iron, (b) That iron is chiefly re¬ 
sponsible for the black coloration, but only to a small degree 
accountable for the swelling, perhaps by assisting to hold the 
sulphur in the clay and perhaps in promoting the early fusion of 
the clay, (c) That sulphur is the final or actual cause of the 
swelling, chiefly by the substitution of silicic acid for sulphur. 

When all three substances are present at once, they are joint 
agents, but sulphur is the worst, and most important cause. 
Clays high in carbon and iron but devoid of sulphur, would 
probably blacken by improper burning, but would not be likely 
to swell. Clays devoid of iron, but high in carbon and sulphur 
would probably swell, without blackening. Clays high in sul¬ 
phur, and containing neither carbon nor iron, would still be 
likely to swell on over-fire, but on the exterior first instead of 
the interior. 

The clearing up of the relative action of these three sub¬ 
stances in the black-coring reaction, and the fastening of the 
harmful structural changes on the sulphur, rather than the stage 
of oxidation of the iron as was heretofore held, is the most 
important result of the investigation. 

Blue-Stoning . 

II. The Blue-Stoning Reaction. Very little new matter 
has been brought out in this test on this subject. It does not 


108 


THE N. B. M. A. COMMITTEE 


appear that there is any persistent or clearly marked change in 
the status of the iron during the darkening or blue-stoning reac¬ 
tion. The clay is a poor one in which to study this reaction, how¬ 
ever, because of its short interval between vitrification and 
fusion, and because of the scoriaceous character of its fusion. 
So far as can be judged, then, Seger’s theory of condensation is 
the most tenable ground upon which to explain blue-stoning for 
the present. The subject should be studied in other clays, low 
in sulphur and carbon. 

•Over-fire. 

III. This clay is not a good one for this subject, 
owing to its very short vitrification range. However, it indicates 
very clearly that the swelling of clays in fusion or by over-fire 
is almost exclusively a sulphur reaction. The gases evolved seem 
to be clearly traceable to the breaking down of sulphates, or their 
reduction by iron or carbon and the subsequent breaking down of 
the sulphites, or the replacement of sulphates and sulphides by 
silica. The latter reaction is probably the important one. Iron 
in itself is not shown to have any direct bearing on over-fire or 
on swelling. The swelling of black cores and of red portions are 
of one and the same nature, and the black swells first, merely 
because it contains much the most of the gas-forming matter, and 
possibly because fused earlier. 

IV. The whole investigation emphasizes the importance of 
sulphur as the cause of a clay’s defects in firing, far more than 
the carbon or the iron. It therefore points to the fact that sul¬ 
phur should invariably be reported in a clay analysis, which is 
now rarely done. It also shows very clearly that the oxidation 
process, or that portion of the burn below 900°C, cannot be too 
carefully conducted—and that in sulphury clays, the period for 
slow steady roasting should be continued for hours after the 
carbon has all gone, and that even then, no matter how long we 
roast the clay, there will be enough sulphur still remain to easily 
swell it to a sponge if over-heated. 

Therefore, for vitrified wares, the percentage of sulphur and 
carbon is the most critical point to be established by analytical 
means, and the selection of a clay low in any form of sulphur, is 
likely, all other things being equal, to give a wide vitrification 
range and a product of good physical strength. 

HD - 142 




































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